When I was looking around for a used tundra three years ago.. Alot of the Tundras I test drove had this problem. I'd say check you lugs!! Make sure there torqued correctly! And you might want to get your tires balanced.
check your driveshaft , I had a vibration and went through 2 sets of tires , finally decided to crawl under the truck and take look at the driveshaft and sure enough one of the weights had come off the the back of the rear drive shaft, I'm taking it next week to have it balanced , I hope that solves the problem.