When I hit around 40mph the engine suddenly revs and I don't go anywhere. If I put it into neutral or 2nd real quick and then back to Drive, it works fine. My question: Is there just an adjustment that needs to be done or am I looking at having to get a whole new (or rebuilt) transmission. (2000 F150 V8 Automatic). Thanks!

 

When I lost overdrive, I could not get it to ingage no matter what I did. When it shifted out of 3rd it went into neutral. I had to replace the transmission. Your problem sounds different.
Post what engine size and what transmission you have and hopefully some of the experts on this site will help you.