I wanted to jump back into the discussion for clarification.
When I said these engines with 4.10 or 4.11 scream above 50 MPH, I want it to be clear that this is normal. With a 3 speed transmission and a load in the back, you must make the engine scream before every gear shift. If you don't, at the next gear shift, your engine will be lugging pretty hard. These engines were designed to scream.
Carl
