Hey guys, I'm new here so please forgive my ignorance. I have a 2002 F150, 4.2 v6. When accelerating, the whole truck begins to shake, around 15 to 30 mph and again at 50 to 65. And today i noticed that the 'service engine soon' light blinks. If I turn off overdrive, it helps a little. I've checked the ujoints, and spark plugs and they're both ok

 

You have one cylinder causing the misfire.
When the CEL blinks it is an indication that raw unburnt gas is passing into the Cats. This you don't want this to happen because the high temps from burning in the cats can melt them internally resulting in much greater expense than just the misfire.
Is there a solid CEL light on? If yes there is a code to be reads that tells you the cylinder causing the misfire. The code will be a 301 to 306 number. Last digit is the cylinder.
If no, a good Scanner with ability to read mode 6 test 53 looking at each cylinder misfire monitor will tell which cylinder is the problem.
The cause can be a plug wire, a boot or a coil.
Look closely at the coil wire locations for arcing and burnt material.
Remove, inspect, replace one wire at a time to check the condition.
It could also be a coil internal failure.
It is not really a hard issue to find and repair with some diagnostics properly done.
Good luck.