Sorry for the delayed reply;
Check your bushings, you may not be able to feel the movement by hand nor see any obvious sign that they are bad without disassembling some. Here's what mine looked like.
I only have 175K but 4 more years. Looking back I've been chasing this problem for the past 5 or more years by slowly convincing myself that every other front end component was the problem simply because they are replaced far more often. The last trip I took, before I fixed it, it felt like the passenger's front tire was out of round and was hopping up and down, varying speed would remedy the problem but the speed that it started or stopped at would vary with road conditions. I like you have new (less than 20k miles) on the shocks.
I will suggest having someone else drive your truck for you while you follow in another vehicle and look to see if you see an obvious vibration or jumping, there is a possibility that you have a belt separating in one of your tires.