Is it still the stock TBI setup? Have you modified the cable bracket at all? I would check the throttle and TV cable first. With any kind of shifting issues on a 700R4, you probably want to check the TV cable adjustment first, and before you drive it any distance. If you mash the throttle at 45 mph it should kick down a gear. At 50 mph it shouldn't. TV cable misadjustment will burn up your transmission.
Drive at 45 mph on a level road. Mash the throttle and it should kick down. If not, stop where you are and tighten the cable a couple clicks and repeat until it does kick down.
Drive at 50 mph on a level road. Mash the throttle and it shouldn't kick down. If it does, loosen the cable a click and repeat until it does. Go back to the first step and ensure it kicks down at 45 mph.
My transmission would bang the 1-2 shift and drive nice in other gears. I drained the fluid in the pan and put in a new filter. I tried adjusting the TV cable and couldn't get it to clear up. The transmission started slipping, and when I pulled the transmission the fluid was black. Two weeks ago I replaced it and immediately adjusted the TV cable. It seems to run fine now.