You did a flush? Did you read through this thread at all? Flushing and changing fluid are different things. Flushing involves actively cleaning out the transmission, while changing fluids is just replacing the fluid in the pans after you turn the truck off.

You may have under-filled. If you open the fill port (the higher of the two bolts) and cycle through the gears with the engine warm, you should see a very fine drip when the fluid is at the right level. No drip, and you should add fluid (and cycle through gears). If you overfilled it, just let it drain from the fill port until its just a trickle.