Sounds like you need to flip both of your diff's over. You can see how I do this in episode 12 of this build when I put it back together and install the diffs. I show how everything turns and how to verify the Diff's. However, being stuck in reverse will not affect not shifting into second. The 1-2 gear is the second gear set in the transmission and spins constant no matter what you are in, forward or reverse. I would back it out until the screw is almost flush with the aluminum body and see if it stays in second, then screw it in until you just feel it bottoming out and it should stay in first. If none of this works, double check that the dog can sling out and is not covered in gunk and also make sure the pins are still good in the hub it slides into.
I have not yet gotten to the tuning of the truck, but that will be coming in the coming weeks. Not sure when at this time though. But I will definitely be covering it.
