2001-2005 are 5 speed autos.
2006-now are 6 speed autos.
There was a 6 speed manual option being te ZF-6.
Vans are a little different with 4L80's and I think the new ones are different...
Bottom line if it is a Duramax (6.6) auto trans, then it CAME with an ALLISON transmission... You can swap in other stuff (why??) but they came with Allison's, no other auto was featured in the TRUCKS.
-Austin- Fightin' Texas Aggie 2001 LB7 2500HD CC/SB 2WD Slower then a three wheeled Prius running on bio-diesel . 546hp & 1025 ft/lbs at Lone Star Dyno, Spring Branch Tx.
Best Run: 13.23 @101 with 1.90 60'