I had a 99 Tahoe 4L60E with both OEM computer and then upgraded to 0411 LS style computer.....so I could tune it and the trans plus add tow haul which the old GMT 400 PCM didnt have.
I found 2nd gear does NOT have lock up at all...(the reason is you can manually put shifter into 2nd and it will start there from a dead stop, and if you were to do that stopped or come to a stop, you would stall the engine). My problem was my highly modified 99 would only pull 45 mph max going up the rim past Payson AZ, nice big 10 mile 7-8% mountain. I was stuck wide open at 4000 rpm in second with my ~7400 lb trailer and the trans temps would start cooking.
Brought my laptop with and tuned the trans with EFI Live to lock up in 2nd gear after 3000 rpms/at certain throttle position/speed. But it only works if you have it in D or 3. I could also change if I wanted it to stay locked up during shifts or not....I cant remember how the OEM tune was anymore. 3rd will lock up as well as 4th and using T/H it will stay locked longer/load.
My guess is your converter clutch may be going, or the pressure to it is too low.
