Sounds like it may need a "quick learn". Any of the trans shops can do it, or anybody with trans software for their scan tool. It will re-learn all of the parameters and clean up the gear changes. If you think it's in the tq converter, you can hold the throttle steady at 40 or so and lightly step on the brake pedal, just enough to turn the brake lights on. That will drop it out of lock up. It that cures it, I'd start in the ignition if it hasn't been tuned up in awhile. When my wife's 99 did what your describing, I added a bottle of limited slip additive from the GM dealer. It cured it, but hers was over 45mph. The lining on the TCC clutch gets hard, and causes the shudder. The additive softens it slightly. These trannys were pretty good after 1999, but they still have problems like anything else.