I Don't know if my prob is the same as yours but i just recently taken a 1990 s15 off someones hands and when i tried to drive it, it seems like it is only in 1 gear the whole way through the gears(second i believe), It wouldn't go into neutral, it wouldn't go into reverse, NOTHIN'.....
So i took the tranny out and guess what I saw. 2 cracked shift forks(not all the way), and one was pretty bent. Also, one of the gears that moves with the shif fork(dont' know the term), was shaved off by the shift fork that was pretty bent......
Hope this helps any. Not trying to scare ya, just letting ya know my experience