Look in the TM library for TM 9-2520-246-34-1. It gives you dissassemby, repair and reassembly instructions for the transmissions as well as transfer cases and PTOs.
Can you tell which two gears you're in between? Where is the shift lever in relation to neutral?
Mine was a worn lever in the tower that slides between the tops of the shift forks as you shift through the gears.
On some transmission models you can take the gear shift lever and the small stub lever it bolts to, out of the shift tower and work the forks back and forth with a long screw driver or pry bar 'til you can get the trans back into neutral.
As a last resort; if your transmission is one that needs to be in neutral before you can remove the shift tower, you may have to wrestle it out of gear with brute force by holding the shift lever towards the gear that's out of place and pushing it forward or back to get to neutral.
Make sure the truck is parked, the wheels chocked, with the transfer case in neutral so there is no pressure on the transmission gears or you'll never get the shift forks to move.
If you can get the shift tower off, flip it over and work the forks around 'til you're able to get them into neutral and the end of the shift lever back into it's position.
If the end of that stub lever is worn, now's the time to replace it or you'll be going through this all over again at some later date.
Once the shift forks are in neutral, slide the gears to get the transmission into neutral too. When you can turn the short shaft between the trans and transfer case by hand, you're there. (The fork's positions in neutral will give you a hint where the gears are supposed to be).
Carefully replace the shift tower making sure the forks drop into their respective slots on the gears.
The problem comes from not following the shift pattern as outlined on the dash plate or in the manual. These things aren't designed to be speed-shifted and trying to go from 2nd to 3rd by shoving the lever diagonally as you push forward (instead of up to neutral then across the neutral gate and up into the next gear) wears out the end of the stub lever.