First things that came to my mind with your problem are possible bad u-joints, incorrect pinion angle, or worn pinion bearing. Although it could be an issue that appeals to the tires as well since tires like 35's have lots of weight to them and also cause similar issue's. Tires would be seen and felt in your steering wheel. Driveline problems will be felt in the floor/seat and heard.

 

ya i had this problem on my 2000 Dakota and it was the u-joins in the drive shaft only happened at 100+ km/h i guess thats like 65 mph

 

With as inexpensive as U-Joints are I think they'll be the next thing I address. The steering wheel never shakes and the truck tracks perfectly straight. Does anyone have a link to a good u-joint replacement write-up?
Thanks!

 

OK. I'll try and keep this short.For 2 years I lived with the same problem. Changed rims-tires, etc. Found a post somewhere that listed 10 things it could be with solutions. i tried it all, no luck. The vibration is not in the steering but will spill your coffee everywhere. Basically if you touch your knee on the door panel while you drive you can really feel it. I was at the point where I was gonna sell the truck. One day my wife snapped one of my 1/2 shafts in a parking lot when she got stuck (winter). I replaced it and all of a sudden next trip the vibration was almost completely gone. Could it be my vibration is actually worn 1/2 shafts? So to prove my point I changed the other side and to my surprise the remainder of vibration was completely gone forever. Boy what a smooth ride now. so if I ever vibrate again, then I will replace them in pairs (Cardone rebuilds at approx 100 bucks each).