Popping out of gear is common on sloppy worn trannys.. my '50 1/2 ton would do that occasionally, but only in second.
My '50 1/2 ton would clang.. but only in second gear. It turned out to be the u-joint inside the "torque tube" deal right behind the transmission.
It didn't clang in faster gears because the RPM's of the joint were fast enough at higher gears that centrifigual force kept it from clanging.
Kind of like an out of balance tire.. the "shimmy" will be more noticable at a certian speed.. the higher or lower you get from that "ultra wobble" speed the less noticable the problem.
Sounds like you could benefit from a tranny rebuild and a new joint from the torque tube to the transmission.
