Ford Repair: F-150 Ford PU, ford f150, plug wires

Question
I have a 1996 ford f150 extended cab with 72,000 miles. I mostly drive it in town and very few long trips. I just had a tuneup, plugs, plug wires, rotor cap, rotor, did a Fog test to break up carbon. Really didn't need that. Tailpipe shows and has shown very clean since I bought it new in 96. Very well kept up.
At 35 to 40 mph, it is jerking alittle or missing but runs smooth at 65mph with no jerking or hesitating. Check engine light is on all the time. A year ago I replaced a broken U-Joint and replaced the other two while I had it on the rack. Had original batterie in it also but had to change it last year. This is a nice kept up pickup and driven carefully. Mechanic doesn't know what to look for. Thanks   Dan

Answer
I suggest you find another mechanic who knows how to check for codes and diagnose codes. If the CEL is on, there will be a code, and it may lead to the problem. If this symptom wasn't noticeable BEFORE this work, then it most likely is related to the work done last.