With the problem you describe, it sounds like your idle speed/mixture is also a bit off. You'll need a vacuum guage and a timing light. Set your timing according to the emissions label, or perhaps a bit more, 14-16 won't hurt. Remember to read the label where it directs to pull the vacuum line from the distributor and plug it. After doing that, set idle speed with the set screw at the side of the carb, using the emissions label to verify the RPM, about 700-800 would be correct. The idle mixture screws probably have cap limits on them, you'll have to eliminate them in all likelyhood. Use the vacuum guage, connected to the base of the carb, to adjust idle mixture to the point where you get the best vacuum reading. Double check rpm, reset if needed. Now double check timing, and verify the correct setting with your RPM adjustments made. This should cure the dieseling problem.
The reason all this is happening is because the idle mixture screws are "limited" in how far you can turn them, which is fine when the truck is new, but not now. So instead of adjusting the idle mixture, they've used timing and idle speed to compensate for poor idle mixture.