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.

Tim,

How many miles are on the engine? If you're using / burning oil it can
contribute to you dieseling by carboning up the cylinders. The cure is a
rebuild.