Truck Repair: Transmission slippage, transmission slippage, gentle fashion

Question
Last night while driving my 1998 Z71 4w drive truck I was going to pass a car in front of me.
I was at 50mph and floor boarded my truck. I noticed that OD did not seem to grab, I then heard the engine just reving high with no increase of speed.
The attemp to pass the car was aborted as another car was oncoming and I did not have the speed needed to complete the pass.
Afterwards I floored the truck a few more times and the same thing, no OD and only enginbe reving high with no increase of speed.
I did notice that instead of suddenly flooring the truck I could ease into the throttle around 50mph and OD seemed to grab but would seem to throw out as I increased the throttle.
Later I tried just easy on the throttle and slowly bring the speed up to top end. Kinda like bypassing OD and just getting the truck to max speed in a gentle fashion. 80mph was my top end speed.
There is NO WARNING light that comes on. Tranny fluid is fine and filter and fluid was changed just a few weeks ago. Engine temp is fine. Auto 1st, 2nd, and 3rd gear seems fine.
A friend suggest it may be a sensor or silenoid problem. Any ideas?
Thanks,
Rusty

Answer
Hello Rusty,
Actually, overdrive doesn't "kick in" like you are thinking. Overdrive is only like shifting into a Higher gear, not a lower one, like passing gear.
Sounds like your tranny might be sucking some air. I am sure there are other possibilities, but auto trannies are not my strong area. Sorry.

Van