Was it a name brand shift kit or "J mod"? Some shift kits for the 4r70w try to mechanically do things that are pcm controlled.
I would not install a separate switch for lockup, likely to cause more problems than it solves. A tune from a reputable tuner will "fix" the shift points and make them much more enthusiast oriented rather than the factory meh that'll get it down the road with the fewest complaints shift schedule.
I personally recommend Marty @ Mo's. I have only had pleasant and informative conversations with him and have only heard good things from others.
My car consistently shifts into 3rd gear and immediate lockup at 29-30mph. I'm pretty sure it's in the tune which is leftover from the previous owner, have a few things to fix/upgrade before Marty works his magic. I do vaguely remember reading something about a check ball that wears in the valve body and can cause early lockup. I could be wrong, I'm still running my spare trans and with the trilogy/4.10s low end torque isn't lacking.
2003 300a Trilogy #? 4.10s
2000 Excursion limited v10
1997 F350 7.3 5spd 4x4 (service truck)
1992 Taurus SHO vortec SC 5spd
1971 GMC 2500 pickup
1971 GMC 2500 suburban 4x4
1967 Camaro (basket case)