My '98 E350 has the shakes. It's not undriveable, but it's annoying, and it makes every interior panel squeek and rattle and generally try to drive me nuts. I got the tires balanced last week and it is maybe a little better, but not a ton.
What next? Balancing the tires has always fixed my vibrations in the past. Where do I go from here? I saw the guys balance it to zero, so I think the tires are cool.
any bubbles in the sidewalls, or perhaps a tread separating from the casing? lose a weight off your driveshaft?
Check tie rods and ball joints as well. Can you tell if it's coming from the front or rear?
nicksta43 wrote: Check tie rods and ball joints as well. Can you tell if it's coming from the front or rear?
I could see those contributing to vagueness(which is present - it's a 185K full size van), but not vibration.
I can't tell which end it's coming from. Feels like all over.
AngryCorvair wrote: any bubbles in the sidewalls, or perhaps a tread separating from the casing? lose a weight off your driveshaft?
Tires look fine. They only have maybe 15K on them, but they are starting to age. It sits for weeks at a time, so I suppose it could be flat spotted tires, but everything I read says that they will get round again within a few miles of driving.
Will a driveshaft weight be enough to shake the whole van?
I used to drive a newspaper delivery van when I was in college... every damn one of those Ford E350 vans would shake like crazy from the front suspension once they got some wear on them. I never had any reason to try to fix one... but it always felt like play in the pitman arm / steering / crazy beams that connected the front wheels area. It would oscillate like the the wheels were turning back and forth themselves and moving the wheel when they hit the stop on either end.
Sounds like the front suspension is worn. Had a similar issue with the f250. Apparently I missed one of the 7 grease fittings.
I would say alignment and if the front end is that worn it could be a big pain in the arse. a drive shaft could also do it.
does that have a one or 2 piece driveshaft? im hunting the same thing in my exploder and think its the driveshaft center support (wont confirm till after finals)
Man, there are so many different KINDS of vibration it ain't funny. 
Driveshaft vibrations usually get worse on decel, but not always; this includes U joints.
Wheel balance issues are generally worse as you speed up. There's another type of wheel/tire vibration where the tires balance out perfectly but as you accelerate you run through a small vibration 'patch' about 8-10 MPH long, that's usually a tire carcass problem. Most times that's just old hard tires although there are a few tires out there (Goodyear Wrangler GS-A's come to mind) that are famous for doing that at low mileages. Sometimes that can be helped (not eliminated) by doing a 'road force balance' but that takes specialized equipment.
If you feel it in the seat but not in the steering wheel it's probably coming from the rear. Bent rim or axle can cause this.
If you feel it in the steering wheel but not the seat it's coming from the front end, look for loose or worn parts. Again, bent rim or axle or sloppy wheel bearings. If it has a steering damper, is it actually working?
Check the engine mounts; this usually is a 'drumming' sound that changes as you accelerate/decelerate and can be load sensitive. I have also seen that to be speed sensitive, much like the tire carcass problem but you hear it more than you feel it. Same thing can happen with a bad center support bearing on a two piece driveshaft but that generally comes with metallic sounds and will be more load sensitive than a mount vibration. Come to think of it, given the age and mileage that's the first place I'd look for a nasty vibration; a worn or collapsed transmission mount.
If there were damper weights on the exhaust and they have broken off, all kinds of vibration and drone can come from that.
Resonance vibration is a whole 'nother subject. 
At work we have a regular customer with a 15 passenger E350 that we see fairly regularly. Seems like at least once a month he's back with the same sort of complaints as you. Every time its in, we rotate and balance the tires on top of checking the suspension components and the alignment specs. It always checks out good, but that doesn't alleviate the issues for long.
What it boils down to is that the heavier Ford vans (and trucks too) aren't designed to be run without a load. The I-beams in the front impose wicked camber changes as the wheels move up and down, and are only reasonably close to where they "should" be when loaded.
Long story short, try getting an alignment on the van when it's at least partially loaded otherwise the specs will be way off when you're actually driving it. Check for chopping on the inner edges of the front tires, and have them flipped inside out if its even slightly noticeable. And definitely start rotating the tires more often than you otherwise would, like every 3000 miles as opposed to 6-8000.
I am chasing the same problem on my E150 van. It shakes more between 65 and 75? New ball joints, tie rods OK, new front shocks, new tires all round, no play in driveshaft.
Starting to think idler/pitman or even the steering box.....
Front end shakes you will feel in the steering wheel as Jensenman said. Ford trucks have a two piece drive shaft with a center bearing. That bearing rides in a rubber isolator. High mileage causes the rubber to get soft and weak. The slightest off balance will cause the drive shaft to vibrate in the mount making the entire truck vibrate. Have you ever run a drill bit in a hole slightly larger than the bit. It will start to bounce around the edges of the hole and the drill, part and your arm will vibrate. Same principle. I chased a vibration in an E-250 for six months before a Ford mechanic suggested checking the center bearing. It didn't look bad at rest, but it was soft. Changed it, and no more vibration.
+1 on aligning Ford trucks loaded. If you run loaded most of the time or pulling a trailer, you need to have that load on the truck when you get it aligned. If you are a big guy like me (265) even your weight will change the alignment. The guy that aligns my van hangs on the front bumper while reading the computer screen. I got 70000 miles out of the last set of tires. No cupping or eating the edges out of them
Bushings could be at fault. Once they start going it lets little vibes feel like big ones, but they're not as critical as they might be in a German car. Check wheels for bends. You can balance a bent wheel, but nothing will compensate for the fact that its not round.
Its also possible that worn shocks are letting the vibes get through easier. I would also suggest you rotate tires and see if it changes. That would help narrow down if its a wheel/tire issue or something else.
Also, if it shakes only when towing, look at the trailer's tires. These things never seem to get looked at until there's a flat. A balance on the tires can do wonders.
There's balancing a tire, and there's balancing a tire. Truck tires are notorious for being difficult to balance. They are plagued with runout, twist, etc. A typical spin balance won't address any of those problems, but one of those load simulating balancers can.
There's also that balancing by placing powder or bb's or golfballs inside the tires. Never done it, no experience with, but I know many other truck owners and aircraft mechanics who swear by it.
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