Engine/Transmission (1998.5 - 2002) nv4500 problems agian

[Hookemupman]

My truck lost fifth gear a couple weeks ago and im going to tear into on Mon,all of my other gears work fine. Let me give you a little input on what has went on . I purchased a rebuilt transmission from standard trans and gear 2years ago the first transmission i got from them lasted about 6 months and started to make a terrible noise inside so i call them and told them what was going on. They said no problem its under warranty so i pulled transmission and swapped out for another rebuilt one. They also do not pay for any labor. So another year and a half goes by and now i have lost fifth gear so i call and talked to them and its out of warranty now of coarse. They told me basically the fifth gear nut cannot back off with there fully splined main shaft that they install when rebuilding and that i should pull the transfer case and tailshaft and see if the snap ring came off the 5th gear cluster. I just dont under stand how the original trans when 145,000 miles and the better updated ones cant get past 10,000 miles each. Sorry but can someone steer me in the right direction to what might be involved in fixing it. Thanks Jason