Clutch Problems

Check the linkage from the pedal to the fork... make sure it's actually moving. This could be as simple as a plastic Z-bar bearing that cracked and fell out.

If the fork is moving on the outside of the bellhousing (about an inch at the pushrod hole), then it's time to pull the cover and check (with a feeler gauge) how much plate departure you have. (Obviously there is not enough because the truck is moving and the gears grind with the pedal depressed fully).