I have a 06 2500 ctd, Auto trans, quad cab, long bed, with 35" tires, with stock gears, so I guess there 3.73, Ive towed my toyhauler travel trailor several times on freeway with no problems, and climbed steep grades on the freeway with no problem, but this last weekend I towed it up in the hills with steep grades,tight turns, so it was very slow speeds, and it was a scary trip. I felt like I hurt trans, And I Felt like my 5" MBRP exhaust with little back pressure was no help, because it made a sound like the turbo was freespinng when I would get to the top of a hill and let off the throttle, and at he same time the trans would up shift very quickly, which didn't make any sense to me, and I would loose all my momentum....That did not seem right at all. And I use the tow/haul feature. Anybody with feedback please !!!

 

Backpressure itself is bad. Velocity is good. scavenging isn't nearly as important on a turbo though.

Slow speeds and steep hills are hard on trannies with heavy loads. When the torque converter isn't locked up, you're generating a lot of heat. What were your trans temps?