I have a 2007.5 Gmc 2500 and I have noticed that when my truck shifts gears from 1st to 2nd, 2nd to 3rd, 3rd to 4th you feel it shift into gear kinda rough and also when it shifts into different gears it slips a little bit from what I can feel. I have noticed that this problem is getting worse and worse. Can it be something in to rear end or the tranny? what do you guys think, input greatly appreciated.

Is this your first ALLISON it is very normal that it feels like its slipping. Not like the old TH400