nighttrain, this might sound strange, but what kind of difference in rpm do you have between 50 and 54, steady speed with off throttle... also, when youre at that speed, lightly press the gas, see if the rpms go up before you feel any accel at all... youre problem is right above the speed where your torque convertor should be locking up

 

roverguy, the vibration happens from around 1500-1850 rpm. Barely got over 50 today, was on a small road towing a jetski. Next time out I'll see how many RPMs it turns at 54mph.

 

also see if you can feel the same thing, might not be as severe, when shifter is in 3rd at the same engine speed

 

same vibration in 3rd at 50mph..pretty sure it's just a wheel out of balance.

 

Once you get it figured out please let us know.