8.8 rear end

Kinda. I get what your saying, but the rear end end is not a true POSI rear end. Its a limited slip rear end. Its designed to have both wheels turning in a straight line equally but only 1 wheel "turn" in corners. But to answer your question Yes, all fox body rear ends internally are the same, all have limited slip
 
POSI-Traction is a GM term.

Ford's term for their limited slip differential is "Traction-lok".

And YES, all 1986+ V8 Mustangs with an 8.8 diff should have limited slip..however by now, most 20+ year old differentials probably need some service.

And GT and LX 5.0"s have the same mechanical, and suspension components. There are no differences.