I have a 1994 XLT(4.0 6cyl auto tran) that is running rough at idle. When I start it up in the morning it is sluggish when backing up or driving, but when it is warmed up it gains a little power. When I get on the road it drives ok but when I get on high speeds it vibrates violently. If I let go the gas pedal it stops with the vibration but does it again when I try to accelerate. I have tried changing the fuel filter, spark plug wires, I cleaned the MAF sensor, then decided to change the MAF as a whole but nothing.

Interesting thing though is that it does not vibrate when I unplug the MAF, I get the check engine but the truck does NOT vibrate at high speeds.

Please assist.

 

turned out to be cracked sparkplugs and an effed up wire..it now purrs like a cat and drives perfect.....I would have never guessed it was sparkplug related since there was no misfire...See below for the culprits..