I still need to figure out my issue - at this point I'm almost positive it's not the tires but I'm not really sure what it is.

If the truck's been sitting for a while (at least half a day or so) I'll get pretty bad shake as soon as I get up to 55-60mph. But, after 15 minutes or so of driving it'll settle down. Still present, but very mild and not annoying enough to worry about. That makes me think it's either all four tires being slightly out of round and heating up as I drive, or something else in the suspension that starts out stiff and loosens up during the drive.

I've had the tires rotated and balanced, road force and all, five or six times over the past year, whenever I was in the shop for other things. Never made a lick of difference. At this point it has to be something further up the drivetrain but, like I said, it doesn't really bother me enough to start dumping money into the problem to try and find the root cause. Just mildly annoying.