This guy brought me his truck and just as the title says he converted his 2008 2WD Silverado to 4WD...The engine is the stock 5.3, he used the stock 2WD 4l60E transmission and the transfer case is from a 2003 Silverado 1500...Everything works perfectly & there are no DTC's at all in 2WD & 4HI....But as soon as he shifts the transfer into 4 LOW it won't shift at all until you let off the gas pedal, but by then you've bounced of the rev limiter about 15-20 times...He took the truck to a couple of shops and they all told him that he needed to have a 4X4 OS flashed into his TCM T-42...I just happen to have a spare T-42 TCM from a 2008 4X4 Silverado I did some work on, so I installed the TCM Licensed it and NO GO! I worked on it for 3 hours, changed just about every parameter in the Tune and nothing, it won't shift from 1-2 in 4LOW...
I'm thinking there's some sort of sensor missing, that the transmission needs when it's in 4LOW but I don't know what I'm looking for...

The transmission has the 20 way connector plugged in to the passenger side. The transfer case has 2 female connectors/plugs on it, one is for speedo and I don't know what other one is for...
Anyone got any ideas? I'll try anything...