91 toyoto Standard shift pick up jumps out of gear (1st, 2nd, 3rd). must hold the gear lever in to keep driving

 

It sounds like your synchro sleeves aren't staying in place. this website explains tranny operation pretty well. It may need a small rebuild.
http://jope.fi/saab/www.quasimotors.com/synchros.htm

 

My 89 4x4 did the same thing, it was the seal between the trans and transfer case, was leaking fluid inside causing the trans to starve for gear oil. After replacing the seal the it took care of almost all of the problem.