Is it time to rebuild my rear-end?

The truck has roughly 387k on it right now.  The rear end is stock.  With how far work is and other things I drive for, I'll probably hit 400k before June.  The mileage alone is probably reason to rebuild it, but recently I've started to wonder about the play in the rear-end

Did some quick clunk-to-clunk calculations while the tires are at the shop getting balanced.

We've got a 6" radius, and from 0 degrees to "clunk" we've got ~1-3/8".  Arc length =  θ × r, with θ= 3.141/180.  (1.375*180)/(6*3.141)= 13.1ish degrees. 
Although looking at that picture, maybe a 16th could be the drum shifting during the clunk, so with that factored, 12.5 degrees

Am I on borrowed time, or is this one of those "it'll run longer 3/4 dead than some do freshly made" situations?

Is it noisy?

There are a lot of things that will make lash with the method you're using to check, many of which can't really be "fixed" like internal lash in the differential itself.

If it's not noisy, change the fluid and keep truckin'.

In reply to Pete. (l33t FS) :

No noise, other than the clunk, clunk when I put it in gear, clunk when I take it out of gear, if I coast down then I clunk when accelerate.  I live in a world of clunk.

That's probably just lash in the differential, if it's not noisy.  Or it could be coming from the spring leaves slapping together or any other number of things.  Chevys are bad for that, Fords I rarely saw it on because I usually replaced the springs at the same time as the spring hangers and shackles when those rusted through.

If the bearings or gears were worn, the rear would be all kinds of noisy.

It might not be a bad idea to pop the cover off and check that the carrier bearings still have preload, maybe spend three or four cans of brake cleaner so you can clean the oil off of the ring and pinion so you can properly check backlash.

Ford is not Chevy though.  Chevy seemed to have used frozen peanut butter for the ring and pinions in their 10-bolts, and I have never seen a Gov-lock that wasn't broken at the diff pin even when there was nothing to indicate that there was a problem.  Ford seems to have figured out how to make gears that last and differentials that don't actively suck.

Until I hear a constant whine, I run it. Difs last a long time if not out and out abused. Even then, they will run noisy for quite a while, again, unless you're doing clutch dumps or going from neutral to drive at 4K rpm.

Whine/grind is when I do it.

Sounds like you have some U-joints that are about ready to E36 M3 the bed.

In reply to Curtis73 (Forum Supporter) :

Nah, U-joints are nice and fresh*

*fresh meaning that I can roughly remember when I installed them.

Pull the cover and do the same test while watching the diff.  How far upstream does the play go?  I have seen them where the axle splines are so worn that there is nothing sloppy in the diff.  When I started rocking the drum back and forth and seeing no movement in the ring gear, I knew I had an issue.  But spider gears, backlash, or several other things could be happening in there.  Only costs you a new gasket and some lube.

Also... I put some "nice and fresh" U-joints in the Impala SS once and 20k later they were toast.  Thanks Auto Zone.  They offered to warranty them and I figured putting more of the same crap joints in wouldn't do me any good, so I ponied up for Spicers.

I have run several Ford rear ends close to half a million miles with zero issues beyond the occasional seal leaking. I don't recall ever servicing them. They just keep going.

If it ain't broke, I wouldn't fix it. Run it until it starts whining, or something fails. 

I personally put 150k on a ranger that was over 150 when I took it over. It clanked every time I started moving f/r. Every time. Warm or cold. That truck is over 500k these days and still going on that same clanky rear end and I'd be surprised if the gear oil has ever been changed once. 
Fresh fluid wouldn't hurt, but I'm not sure I could be bothered if it was mine