I would also guess that the head gasket has been dealt with, because I dont think they last that long without fixing. Or.. you have a BHG already.. but thats a guess. Just going by the mileage.
The differential gives me a little scare though because you haven't really pinpointed the problem (or did I miss something). Good price for the car, but I generally fear the unknown. The rearends on these cars are pretty well indestructible from what I have read, so I just wonder what the specific issue is and how it came about.
(remember.. Im a Supra owner.. not a mechanic... thats Mr. Nickleye's forte!)
Also, I have only owned my Supra for about a month and a bit... and already, I have found alternate cars that I would have bought if I didn't buy this already. Dont move too fast into one of these just because they are tempting (damn fun to drive!). There are lots to look at. Having said that... if the diff isnt really messed up, sounds like a good car.