Hi IRNCTY - sometimes this is caused by the little detent balls that travel back and forth in a drilled passage between the shifting rods. The balls stick into rod grooves so only one rod can move out of neutral at a time. But as the rods and balls & passages wear, the accumulated clearance can allow two rods to move at the same time, putting the transmission into two gears at once (just a lock-up if you are lucky - gear tooth breakage if not).
Well, does that mean it needs new rods and balls? Are the balls under the tiny little springs that come all the way through and are held down by the shifter plate on top? So far I've only had lock up.
Thanks for the reply