I have to agree with Tech on this one....start with the engine & work back. Sounds like tranny to me- but that is just a long distance guess.
I had a Ford PU do that to me once, I thought that it was a blown head gasket because of the loss of power, but it ran fine at a idle, the end result was that the 2nd & 3rd gear sprockets had self welded themselfs togather and it was trying to pull both gears at once. I had never seen this before - so when it happened I was stumped- bad.
I found it by doing what Tech has advised you to do......
Good Luck & let us know what it turns out to be....