I agree with shade. Until you pull it apart and split cases we are just guessing. How does reverse work? There was a recall on the 05 on the reverse interlock return spring. Being that you have a 2013 it will already be there but It is possible if that spring broke it could cause shifting issues. And there are a few instances i have heard of that that spring issue caused second gear issues. Not entirely sure why. Concidering reverse is one down from 1st gear. But i do know when that whole recall thing was happening they would go into reverse without turning the reverse lever. Just a thought. And thank you for your service! When you get back and start the build feel free to hit us up for questions.