I lost 5th again, 25,000 miles ago I paid to have my transmission rebuilt with the fully splined shaft, hoping to solve the problem. Well, took it out and like I figured bone stock parts, not much I can do as my receipt is very vague as to what was done to the transmission. I can guarantee it won't happen again though, brand spankin new NV5600 is in there now. I've got some learnin to do, they put reverse in the wrong spot. 
