I had something similar to that on a different truck. It turned out to be a ply separating inside the tire. The warmer it got, the worse it was. You may need to do some tire rotations to isolate it if that is the problem or take the tires to a shop to check balance. Mine would balance then go bad. They would rebalance them with a lot of weights in a totally different spot. They finally admitted to the defect.

1997 T-100 4X4 Xcab,Warn Hubs
02 Camry LE
2008 Yaris