Cab & Seat Shake

Driveshaft??

 

Pretty sure the only way you can check a driveline for being out of balance is take the complete shaft right off with hanger bearing and have it balanced

 

If you have dual leveling valves on tractor and one is going bad it can cause chasis vibration and is the last place most shops will look. After charging for all the things they check and "fix" first.

 

Have they checked drive-line angles?

 

I had a similar issue with my 04 and it would shake at 45mph. I thought it was the driveshaft too as the steering wheel did not shake at all and rode straight. Eventually I noticed that one of my steer tires (cheap China brand) has some odd wear and I put a set of bridgestones on it and now that vibration is gone.

 

Crapy tires wear uneaven, I usualy see my cab shield go up and down at low speed and then it vibrates at 50

 

My 08 4900 ex i finally cured of the shakes ( well almost) it took heavy duty frame liners, cab mounts and motormounts to fix it. Truck has only been in service for 1 year so warranty covered it for me. 7+ weeks of downtime is no joke though.
I have the c15/13speed and 3.42 gears. Would shake like crazy between 55-62. Frameliners and new cabmounts seemed to fix that.
New motormounts took care of vibration under load in low range and going from coast to pull. Currently running 22.5 supersingle drives to save weight since the liners added 500 pounds
Good luck!

 

tires, even trailer tires will and can cause vibrations in the cab.