Won’t shift out of park

Hey guys, I own a 90 k1500 z71 package few months ago was changing out ujoints found out my rear end has been replaced (with what not sure). Last week parked on a small
incline and I had to jack up the rear end in order to shift into drive. Yesterday I had to have it towed home because jacking I up the rear end didn’t work. Does anyone have a clue as to what my issue could be?
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