i had the same problem on my transmission.
first, it wouldn't go into reverse for about a week, until i put it into low gear, went foreward a few feet, and then revved the engine and finally jammed it into gear (very softly). after that, a month later, it got jammed into first gear on my friend's ranch, but the shifter was in neutral. it turned out that a little ring in the tail-portion of the transmission got lodged and split between two gears, so I put a trans. out of an '89. if your trans cross member bolts are on the set of holes on the frame closest to the front, those transmissions (shorter by about 3") tend to ***** up every once in a while before crapping out on you. the newer ones (rearward holes on frame) don't lock and stuff, but the one I have has some pretty sh*tty synchros. just look at the frame where the cross-member bolt up for this pattern:
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* * *
* *
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there should be 3 rivets and 2 spare holes