solar,
yep, if you can manually push it and the truck runs ok then i think it's safe to say the contactor is probably worn out.
You can get a rebuild kit but by the time you do that and rebuild it you can just get another contactor assy and the cost would be negligible and quicker.

This happens when they get worn out, the tips can get compressed or burned down, springs get weak, bushings get worn causing the armature to drag, several things can cause it not to work right. the tolerances on the contact tips sometimes are not as much as one might think, they might look fine but if they wear off or compress enough the travel bottoms out and they just do not connect good enough naturally. You proved that by pushing them manually and the truck ran. So either rebuild it or replace it. Just FYI, the fwd/rev contactor assy are one unit build on a single chassis so you can't just do one side or the other, both sides of it have to be done together, if you've never rebuilt one before then i would suggest just getting a replacement to save you some headaches. It's like a puzzle that has to be assembled just right or you can damage it unintentionally.