Sounds like a vacuum issue.
Try this:
With the truck running, take a can of starting fluid and spray it around the intake manifold and hoses, taking care not to direct the spray into the carb.
If the engine increases in speed, check that area to see if there is a hose that could be cracked or a plug or seal missing from the manifold. If it kicks up RPMs around the manifold, you'll have to replace that gasket.
If not, you should have a nice clean manifold when you're done!
TR