Can you put the truck in first when off and turn it on and get going? Can you do the same in secong and third? If you can, make sure you are not holding the lever. If the trans throws the lever out, you have a bad synchro. It all sounds to me like either the pressure plate, clutch, or throw out berring. Try the top experiment to see if it can be anything else. I have abused my trans missing gears here and there and I've never had any problems with the synchros.

1985 Toyota 4x4 4Runner. 1989 Suzuki Sidekick 4x4.