Transmission slipping on 2006 Silverado

[ChevGuy12]

I have a 2006 Silverado 1500 4.8L with a 6" BDS lift & 35 in. toyo tires w/ 20" rims on the truck (had a bully dog programmer on but had to remove). I just put on iron cross bumpers (front & rear) on the truck, the weight of the bumpers made the truck a little sluggish and made the tranny slip out of 1st gear. The next day I took the truck the truck to a transmission shop and had regained back 1st gear when i started up the truck to drive it to the shop!? Any comments on why and what maybe i should do (change the gear ratio, get another programmer, upgrade tranny) ?; I ended up getting new gears found a calculation that should get you near to what type of ratio you need for your vehicle. I had originally had (factory) 3.42 and r/r with 3.73's.