You need to check what size jets you currently have in the carb. It may be setup for high altitude. The carb probably needs a rebuild also. You may also have a vacuum leak at the intake or in the myriad emissions components that is causing the rough idle. This is the part where you either become a shade tree mechanic or prepare to start spending $$$ to keep the truck on the road.
At least the shop admitted they don't have someone who can work on your truck. Most would probably take your money without any idea how to resolve the issue. Get a Factory Service Manual if you don't have one and dive in.
