I have been looking into this a little more lately...and I think I might do it...while my car is down for the engine rebuild.

Problems:

Axles are different
Rear cover is different(Although some holes match up)
driveshaft flange is different.
front mounts are different

So really...this is not a fit at all, but could be made to fit.

I know I could get some custom axles made up(too bad I junked a supra with perfectly good shafts)
GS rear cover can be modified to fit the supra diff.
Going with different tranny with 4 bolt flange anyway, so I can have a 1 piece driveshaft made.
and the front mounts...I will just have to make a bracket...not a big deal, maybe a couple hours of work.

Yes, seems like a lot of work, but other than the cost of the axle shafts, this will be a faily cheap swap. I really wish they would have used the same flanges for the axles...this would be a lot easier. I will also have to look to see if it is possible that the MKIII axles will fit(probably not, but you never know)