If it shakes while sitting still, in neutral, that is an engine miss. When was the last time plugs/wires/cap/rotor were changed? Is you plenum gasket blown?

 

That narrows things down quite a bit. Either a weight has come off your flex plate or you have a miss in the engine.

 

I was going to say transmission mount….mine just went cuz rubber separated completely from the metal so it was floating around and causing similar issue

 

How do I know for sure if a weight has come off the flex plate?

 

Under the truck, right where the engine and transmission meet, there will be a little plate to protect the flex plate from the weather and road debris. There should be 4 or 6 little bolts holding it in place. Removing them will let the plate drop. You'll notice little weights around it. The method of balancing changed around then but where the weights are welded, you'll see some welds that aren't holding anything. Other places they will be holding a piece of metal.

It's not very common for the weights to fall off. You may just have a bad plug wire or an injector not working properly causing a misfire.

 

Think ya need to pull the starter to remove that plate as well, but, it's been a while, so, I could be wrong.

 

You may be right. It's been quite a while since I was under one of these and then I was getting ready to pull the engine/transmission assembly.

 

HeyYou is correct starter has to be pulled