I am experiencing a crazy shake through the whole truck when I am towing an enclosed trailer loaded with about 1000 lbs. When at low speeds in town no problem. I start to accelerate, with or without tow assist on, when it shifts from 3rd to 4th gear and the RPMs drop, the truck shakes like no ones business. If i do 55 mps I don't get any shaking, but the truck does not want to shift to 4th gear and if it does I don't get the shake like I do when trying to go 60/65. Truck seems to drive fine with no load on it. All wheels are good and balanced and I have checked for bad motor mounts and transmission mounts and my eyes see nothing, or at least no cracks in the rubber. I have no play in the CV joins and I have one boot that looks a little warn on the outside but again no play when I yanked on them. All plugs and coils are new, trailer has been towed by other trucks of mine with no issue. I am al a loss. please let me know your thoughts or ideas.

 

What plugs did you install?