Yes, the nice thing about the 420 is that you can take the whole shifter and top plate off without even needing to drain the oil. Take the floor plate out of the cab, remove all the bolts around the perimeter of the top plate, and take it out. The shifter forks and mechanism come off with the plate, and you can try to move the gears by hand in the tranny. The big sliding straight cut gear is 1st/n/second, and the little lever on the right side is for the reverse idler. The other sliding clutch is for 3rd/n/4th. You should be able to click everything into neutral by hand and see if the clutch is actually working or not. If it was winter I would say that you got water in there and it froze up but that's not the case. Do look for water, though, it's a pretty common problem.
