I think you need a new clutch on the second gear. Find a parts diagram for your truck and dissassemble the entire gearbox and look for anything that doesn't look right. The symptoms you are posting sound like it has nothing to do with the reverse function at all.

On second thought, there should be some sort of grub screw or bolt that holds the pawl clutch onto the shaft. Maybe that has been damaged somehow? Anyway, thats where I would look first, and make sure none of the gears are worn down.