[rolymon]

Giving this further thot; You need to isolate the problem to determine if it is the clutch or not. With truck at rest and parking brake set. Start engine, put in neutral, release clutch, push clutch, wait 2 seconds or more, and shift into reverse. Does it go in to reverse without the sound of grinding gears? If there is no grinding gear sound, it probably is not your clutch. Reverse is non-synchronized. After you push the clutch, the entire gear train in the tranny should come to rest if the clutch is disengaging completely. The forward gears are synchronized. If the clutch is not disengaging completely and you attempt to shift into first gear at rest, the gears in the tranny will still be spinning, partially powered by the engine, and the synchronizers will attempt to synchronize the gar speeds but will be unable to do so since the gear is still under power thus the shifter will feel really stiff preventing the gears from easily engaging..
If you get no grinding, the clutch is OK and then you need to explore the transmission. Other posters have provided other ideas including mine about having the right weight lubricant in the case.