Sounds like a bad synchronizer. You may also have a bad bearing.

I'd first make sure that your clutch isn't dragging when it's disengaged. Put the vehicle on a flat surface, brakes off, transmission is 1st gear with the clutch pedal depressed. Is there any tendency AT ALL for the vehicle to move forward? If not, I'd eliminate the clutch as a potential cause, and concentrate on the transmission itself.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.