Another thing to check is the fuel pump. I just replaced a fuel pump that would not deliver a constant fuel supply under heavy load but when the van was up to cruising speed it worked fine . The pump couldn't produce the pressure necessary for acceleration and gave a jerky takeoff at low speeds(higher rpm) but smothed out at higher speeds(lower rpm) when it didn't require as much fuel . Check the pressure on the fuel rail when the van is not running. Turn on the ignition and listen for the pump to hum and the pressure should be around 40-50 lbs. When the van is running it should have around 35lb. This video is a good explanation of how it works and how to check and diagnose the pump .Hope this helps