GMC Repair: engine missing during acceleration, gas gauge, gear changes

Question
When I accelerate from 60 mph to 70mph my truck begins to shake until the gear changes.  Most tell me it's a tire out of balance but I don't think so because it doesn't happen unless I am accelerating.  Also I've noticed my gas gauge is jumping just a few millimeters back and forth.  What do you think?  THis is a 1999 yukon with 120,000 miles if that helps

Answer
Kelly

Sorry i didnt see your other question,i see that you have the year make and model.
Let me know on the acceleration problem if it does this from a stop also,it could be a few things,either spark plugs,ign wires or a fuel injection problem,if it is acually a missfire under load.let me know

ED