My truck is a 2004 2WD Quadcab with the 4.7 and 5 spd automatic, 3.55 gears, 55,000 miles. In cold weather, say below 25 deg F, I get a lot of engine vibration that isn't present in warmer weather. The whole thing just sounds noisier and, for want of a better term, feels "rough", so much so that you can feel the vibration in the seat of your pants. It's not like a misfire, more like the engine is mounted directly to the frame with no motor mounts. Not RPM dependent, pretty much at all engine speeds, at least up to 3K RPM. I'm running NGK plugs, and plan to soon change back to Champions in hopes that will help.
Anyone else noticed this phenomenon or know of a cure?

 

May just be the cold weather making the motor mount bushings stiffer? If it was poor combustion, it would clear up when engine warmed up.

 

Last things first-- it doesn't clear up when the engine is warmed up, even after 30 miles of driving, although it does improve a little.
As to the mounts getting stiffer, well, I've thought of that, but it seems the mfr would have the foresight to use a rubber that wouldn't change that much. Also, it would take a lot of stiffening to give the amount of inflexibility for this much vibration. Could it be a belt vibration caused by a similar hardening of the rubber?
I'm totally at a loss for an answer; thanks for your reply.