I bought a LSD rear end on ebay and it was stated that it was a 84-85 GSL-SE. Well, its a '83 GSL. It fits in my '85, but the splines in the axles of the '83 are smaller than the 85 so my '85 axles don't fit.
Can I use '83 axles in my stock (base model with drum brakes) '85 rear end so that I can use the '83 LSD?

 

Nope, 84-85 are large bearing axles and 79-83 are small bearing axles, meaning the housings, 3rd member and axle shaft are different.