Check your slipper, slipper pad and clutch in the clutch bell. If your slipper is slipping bad, when it does engage 2nd gear, the slipper just slips vs the truck going faster and the motor dropping in RPM. Once it slips, it glazes the pad, then it slips worse and worse until it gets so hot it melts the center out of the spur. Similar issue applies if the clutch shoes are worn down to nubs, got oil or grease on them.
Could also be the 2 speed adjuster/pawl is damaged and not allowing the pawl to kick out. Depending on how old it is, they used to have a really weak peg inside that would snap off. Would usually cause severe damage to the 2 speed gears in the trans when that happened though and it would bind up due to the chunks banging around in there. They updated it quite a few years back to have a more solid plate/piece for the pawl to hammer into. Before the plate existed, we would put in a second pin (came stock with 1 pin) for the pawl to engage into, so it wouldn't slam into gear as hard.
Could also be that whomever had it before you could have put a second grub screw over the 2 speed adjuster grub screw to keep it from loosening on it's own and your not adjusting anything, your only turning the screw that locks the actual adjustment grub screw.
If it's the post-X version, you should be able to take the top of the transmission off to take the 2 speed gears out without dismantling half the truck. It's a few screws, take the top of the trans off, then remove the 2 speed mechanism to inspect it.