My local diesel shop but they outsourced the custom tuning. If you don't have a diesel (can't remember if yours is?) then you can buy a unit and just adjust tire size or maybe even just take it to a shop to have the adjustment done. There are a number of much more knowledgeable folks on here than I who can likely chime in. I will say this though, there are a lot of folks running oversized tires on their rigs with the 5R100 trans with great success.
2015 Chevy Express 3500 Duramax
w/ Quigley 4x4 & Agile Fox shocks
Sold 2005 E350 Chateau
Quigley with Agile RIP, 6.0 PSD