Under load or moving the truck requires more fuel, therefore I would check the fuel line, fuel pump and inline fuel filter to ensure there is no clogging or interuption of good fuel flow into the carburetor.
I had one that acted like that before and the fuel pump was on the fritz. A new fuel pump later and she ran like a champ. I have also had similar symptoms, only to discover the coil wire and a few of the plug wires were not properly seated at the distributor cap and spark plugs. When changing plugs or wires, I put dielectic grease on the connector boots at the plugs and the cap, to ensure a good connection.
I hope you are able to solve your dilemma.
Tony
