My knowledge is limited to the 3 speed non synchro low trannies. There is one synchro gear that rides on a splined locking carrier. One shift arm moves the synchro, the other moves the carrier in the correct symphony to select the gears. If you can see inside the tranny with the side cover removed and tranny still installed, then pull the sidecover first. Disconnect driveshaft, drain oil and pull sidecover and linkage. The shift detents are controlled by spring/pawl action in the sidecover. You can inspect sidecover for damage, then peer inside tranny for grenaded gears or just a stuck synchro/carrier. Look for chipped teeth or metal shavings in oil or bottom of tranny case. The reverse idler gear or the 1st-2nd cluster gear are the ones I used to frag all the time. Its normal to see some slight deformation of the synchro gear teeth where they mesh with other gears during operation.
You may be able to push synchro/carrier back into neutral with some careful application of screwdriver to gears. Sometimes just removing the sidecover and re-orienting the parts will fix the problem. The trannies will hang if the shift forks/pawls get "confused" when the linkage F&^%s up. Then reinstall sidecover and shift tranny by hand. You may need a helper to hold in clutch for some of the tests/adjustments. Grab output shaft and try to spin tranny in neutral and all gears with clutch disengaged, feeling for rough spots or weird noises. You can also reassemble tranny with new shift arm, and leave driveshaft disconnected. Start truck and go through the gears at idle (in cab!) while listening for damage/weird shifting.
Of course if you find excessive wear on side cover shift forks or metal in tranny, pull the tranny and put it on a bench for closer inspection.
good luck
chip
