Truck has just under 1/2 million kms. I have a vibration in the seat of my pants or behind the front seats. I have ruled out tires, shocks (new), drive shaft. It occurs around 100kms & shakes the truck. Smooths out around 125kph. Starting to drive me nuts. At it's worst it really shakes the truck. I asked my buddy to watch the back of the truck from behind while we were on the highway to see if the truck appeared to shake when this was occurring. He said no. Any ideas? Thanks

 

ball joints, u joints?

 

Loose engine or tranny mounting bolts.

 

Look for the exhaust contacting any part of the body or frame and transmitting vibration. Make sure the exhaust is being suspended by all its proper rubber isolators. Give the tailpipe a shake and make sure it moves somewhat freely without bumping or scraping anything.

 

I had an old 'burban that had the axle bearings/axle face itself get pitted. Caused the same results- and yes, I also balanced my tires/drivelines ect. before I found it!...

 

Your are not talking about the Dodge death wobble.
You said when it is bad it is really bad.
I have have a few 2500 4x4 that would get some bad vibes in the front end and sometimes would build up to a REALLY big shake, strong enough to throw drinks out of the cup holder.
I balanced tires, new front D-shaft, trailing arm bushings, tracking bar and all the other wear items in the front end.
I finally changed the steering stabililzer and the wobble went away for 60,000 miles, when I threw another one on.