2006 2WD. My trans has not shifted properly since shortly after my LS3 swap. At first the shift from 1 to 2 would get firmer as I drove the truck, until after several miles it would bang into 2nd. I would shut the car off, and the 1-2 shift would go back to normal for a while, then after several minutes would get harder and harder again. After several thousand miles, the whole "trans doesn't want to shift out of 1st, and when it does it shifts to 4th" thing started happening. The truck had to go to a tuner to get the ECM to work with the LS3. Someone else on here alluded to the fact (back in 2008) that their tune may have caused shifting problems and that a re-tune fixed it. Is this true? My tuner says he had nothing to do with it.