GSL/GSL-SE axle housings interchange. The only difference in housings is between small bearing/big bearing, and disc or drum. (So four total housings out there)
My car currently has a GSL-SE housing and GSL internals. Soon I'll have the GSL-SE axles in it too, just need to find some more wheels with the correct bolt pattern. There's nothing wrong with the 110mm wheels if you can find them, but they are worth so much money that if yo have them, you can convert to GSL-SE, sell the 110mm wheels, and still make a profit. 
Beware of the rear brakes, as the GSL-SE have vented rear rotors and the GSL non vented rotors the calipers will not fit. Also the GSL wheel has an offset of 17 whereas the GSL-SE is 38 and the axles stick out farther due to this.
Also the larger bearing rear ends commenced being introduced part way through 1983. So some 83 rears will be suitable.
I have two GSL-SE rears which some say are poorer LSDs, but in reality the clutches are positioned in a different manner only.
My car is a mid-83. How does one go about figuring out which rear end is in it easily?
You have to remember though that the 1984 cars were built starting in June or July of '83. So they were in the middle of '83 but they were 1984 model year. Mazda had to accommodate the huge lag time between building the cars and actually getting them in dealerships most of the way around the world.
If you want to make ultra special extra sure, which is a good idea since it is 2009 and your car has been old enough to drink for quite some time, and has a good chance of NOT having the bits it was built with, you can measure the diameter of the rearend housing at the ends where the bearing retainer/backing plate flange is. I don't have a big bearing rear handy to measure but the small bearing housing is approximately 2 3/4" OD right next to the flange. I measured a '81 disc and a '79 drum housing to be sure they were both the same.
Another way that only works if you're comparing disc rears is to look at the bolt pattern at the flange. On the small bearing housings, the caliper mounting holes and two of the flange bolts are on the rear of the housing, and one flange bolt is on the front. On the big bearing housings, the flange bolt pattern is reversed, with two of its bolts on the front and only one on the rear. Mazda probably did this to keep the flange bolts away from the caliper, probably. (Drum rears have a 4-bolt pattern, and I have only ever seen small-bearing drum rears up close and personal)
No I mean it's actually a mid-83, as in half way through the production run. There's some other odds and ends that changed during the model year run and mine has been "early" once and "late" once on replacing those. It does have the original rear (bought the car from the original owner with all service records).
I'll pop a rear wheel off and take a gander at the caliper mounting to see. Mine still has the original metal tag on the housing too if you can run #'s. I'm pretty sure it's a small-bearing rear from when I did the driveshaft munch, but it doesn't cost anything to check it out and see.
if it is a 83 it has the old dash and interior, I had one of these series 2 cars with the larger rear end, series 3 84/85 all had the newer dash
Ignore the driveshaft. '83 was a weird year. It had the coolant-fed oil cooler like the later ones, but the wiring harness went to the engine on the driver's side like the early ones. It (should have) had the small bearing rear, but it also (should have) had the later style driveshaft with the symmetrical bolt pattern and non replaceable U-joints. (Early shafts could only bolt on two ways)
'83 was a transition year.
Complicating matters, a lot of people converted their cars to early driveshafts and oil coolers. Or the opposite: I have seen early cars with beehives and the crappy late driveshafts. Why? Who knows?
It's an 83 Model year, built in 83. It is a Series 2 (old dash). It has the beehive oil cooler and driver's side harness. It (had) a later style driveshaft with the non-replaceable u-joints. It now has the asymmetrical bolt pattern large flange, replaceable u-joints, and an older driveshaft.
Like I said, I'm 90% sure it has the small bearing rear, but with 83 being a transition year I might as well check it out.
The early axles were ~1.060" x 24 spline, that's what I have in my hybrid rear axle (RX2 housing, GSL limited slip, 4:86 Mazda Motorsports R&P). I have the larger style stub axles in the Abomination (2nd gen RX7 diff) and it's assembled so I couldn't measure. 
I think those are 1.120" x 26 spline.
