I'm inclined to think either a lose slipper clutch, or your flywheel came loose. Try tightening the slipper; it should be locked down all the way, then literally backed off a 16th of a turn. Doesn't take much.
If the slipper did come loose, search for the "slipper coming lose - the fix" thread.
Next, with the engine not running, hold the spur and try to roll the truck BACKWARDS. It shouldn't want to roll. If it does roll, your first-gear one way bearing is slipping/shot.
If it wasn't the slipper or the one way bearing in the tranny, fire it up and just watch the flywheel. Give it a gentle but short revv of the engine and see if the flywheel and clutch bell move.
Try those three things, and post back.