[Dave at Pelican Parts]

If the problem is all the forward gears, it is most likely the clutch.

If the problem doesn't occur in reverse, it is most likely not the clutch.

Reverse gear is the lowest gear in the box, so it may be that there is still some slippage in reverse but you get enough torque multiplication not not notice it.

I would suggest first checking the clutch adjustment. Then the shift linkage adjustment and checking the bushings. If none of those produce results, you're going to have to pull the gearbox. Happily it isn't super hard to do. The clutch will be very visible at that point, plus you'll have access to the whole gearbox.

--DD