Hesitates and low power in gear

Engine was replaced at 177k miles, along with rebuilt trans, fuel tank and pump and lines.

Truck runs and starts up just fine, idles just where it should be. Once you place it into drive or reverse the truck "falls on its face" drops down to 500 or below rpms. Stumbles and has very little power when hitting the accelerator. Every so often while drive the truck will jump up and run just likes it's supposed too then fall back on its face.

Any help would be appreciated.