GM figures they can't charge enough for the twins to make an Allison happen. That and the HD folks may not want to share. Just throwing that out there.@Dmax Dan I totally agree about the Allison option. Wish they had something made by Allison in our trucks
For your truck transmission issues though: have you changed the transmission fluid and how many miles are on your truck? I don't tow much; our 6 x 12 utility trailer is all I tow when I do and it never gets loaded down very heavy at all. I do went with hACKman Customs tunes right at a year ago and at the same time (46k miles) went and changed out the transmission fluid and rear diff fluid with AMSOIL fluids. Didn't have any issues with the transmission before that I recall, but I felt it was time to change fluid. I now have almost 78k miles and still have no issues with either. I plan to change the trans fluid again at 90k (and do the front diff).
Might be worth changing fluid if you haven't, and maybe use a quality fluid too.