That's not good. It's impossible to diagnose from here, but it sounds like that transmission is a goner. It might be a fixable problem that doesn't require a rebuild, but with flushing changing the symptoms like that it's probably rebuild time.

It's best, if you need a rebuild and intend to keep the truck for a long time, to go with something better than a stock rebuild. If the cost isn't prohibitive, the folks at PATC build some darn fine transmissions. They also sell parts to more or less duplicate what they do, if you can do it yourself or get skilled labor for cheap.