3 Answers

The synchros are worn out and need to be replaced. Not an easy task if you have never done it. Call a tranny shop and find out how much it will be and then decide if it's worth the money to replace them or just get a new tranny. 

If it's stuck in gear, if it's on a slight grade sometime manual tranny will get in a bind. An old trick is:

EMERGENCY BRAKE ON.  Have a strong someone place a 4X4 about 5' long under the rear bumper and someone inside. Person in the back pulls up on the 4x4 causing the strain on the transmission to release and the truck will move forward an inch or two,  at that time shift back to neutral. 

I had an old Dodge years ago, I carried a 4x4 in the trunk, it was always getting in a bind.

The linkage on the clutch may need adjusting...or the cable,(if equipped with a cable), may be stretched too much to adjust and may need replacing...Good Luck!!!