Need help quickly

Okay a little back story, I have a 2000 Ford Ranger 4.0 4x4. It has the 5r55e transmission. The transmission was recently rebuilt with the upgraded valve body. However, the truck will not shift from 2-3 under acceleration. If I let off the throttle, the truck will shift and continue on.

The interesting thing is, it only happens when the engine and transmission are warmed up.

I originally thought that the mechanic that rebuilt the transmission failed to use Mercon V transmission fluid. I assumed that the cheaper fluid was foaming or breaking down when it heats up. Therefore, I added some Lubeguard© additive that was supposed to turn it into Mercon V. I had little faith in this but it did seem to work for about two weeks. Now the problem is back.

Now for the kicker. I swore once I got this transmission working right I would sell the truck as fast as they could drive away with it. So I listed it on Ebay. The auction has 3 days left. Therefore I have 3 days to come up with a solution or I can take a loss on the auction. Clearly I don't want to lose money.

I will take any suggestions. At this point a blind guess is better than nothing. I'm between a rock and a hard place here and whole heartedly appreciate all the help I can get.