I just starting experiencing this exact same problem, it seems to be the worse at exactly 70mph, > 70 seems normal and <60 feels totally normal. If I'm accelerating I can barely notice it through 70 but If I take my foot off the gas it is more prounounced around 70mph. I just got back from a trip to the sierras where I hit a snowbank at ~20mph, so I just hope I didn't damage anything there. My tires are pretty new and all have even wear , but I guess something could of gotten unbalanced, but the truck still drives straight when I take my hands off the wheel . This was also the longest trip ~800 miles I've took the truck on since I have owned it. I also was using 4wd low a little bit also if that could possible have anything to do with it.
From a quick inspection of under the car everthing looks normal except for a lot of grime on the inside of the front wheels.
I also starting getting a check engine light about half way through the trip ??? , but that's another issue...
I'll be sure to post back if I figure it out, I might have to take it in anyway to see what's the deal is w/ the check engine light.