Front axle

I was wandering if the front axles (left and right) for a 1990 Audi V8 3.6 Automatic with UFO brakes are identical to the ones on a 1992 V8 3.6 automatic equiped with the Girling dual piston caliper and the different strut assembly. Also, do you think I should worry and replace a rear Thorsen differential that has been mildly rumbling for the past two years (fluid level OK, changed to synthetic 75-90)? I have the opportunity to get a used rear differential for about $50.
Cheers,
Florin