Have you put new sparks plugs in it. If you havent I would try them first. Only run the ac delco plugs listed for the truck. I have had many people come threw the shop with misfiring and trucks running like **** and come to find out they put bosch or ngk plugs in it. Your manual says there good for 100,000 miles but there really only good for about 75,000 or so. Also run some lucas fuel treatment or b12 chemtool. both you ad straight to the gas tank. If it calls for one can for 20 gallons buy two and double the treatment it wont hurt you engine.
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Misfire could be a number of things: a) spark plugs - change them if used over 15K miles - inspect/regap. b) spark plug wires - change them if over 15K miles - inspect c) ECM (electronic control module) - test with HEI (high energy ignition) tester.
The shaking tells me that the engine is running on less than its normal compliment of cylinders - the shaking is produced by the imbalance caused in the engine by one (or more) cylinder(s) not firing at all.
The "Check Engine" light probably is coming on due the the misfire condition. Go to Pep Boys or Advance Auto Pats, where they'll tell you (free) your codes causing the "Check Engine' light to be lit - the code coming from your truck's computer will probably be one or more of: P0300 - Random/Multiple Cylinder misfire detected P0301 - Cylinder 1 misfire detected P0302 - Cylinder 2 misfire detected P0303 - Cylinder 3 misfire detected P0304 - Cylinder 4 misfire detected P0305 - Cylinder 5 misfire detected P0306 - Cylinder 6 misfire detected P0307 - Cylinder 7 misfire detected P0308 - Cylinder 8 misfire detected P0313 - Misfire detected with low fuel
Does it miss at idle or going down the road, maybe both? Any applicable trouble codes? If you have that spider fuel system, I usually suspect that first. That system is noted for problems.
You could try a cylinder balance test, engine idling, you could kill the cylinders one at a time, quickly. If you kill a cylinder that has no effect on the idle, the cylinder is dead. Then you have to decide if problem is spark--fuel--compression on that cylinder.