Mine has recently started doing this also... it is not the Death Wobble that other speak aobut just what feels like front or truck is hopping like on a wavy road. It is there at exactly 46 MPh... but goes away at higher and lower speeds and i have noticed it does not occur at all on concrete paved roads.. strange. Will be digging into it this weekend to see if i can isolate where it is coming from. I have some worn body mounts, and maybe some shocks that need replacing (70K on those)... but have new tires with only 10K miles. Will let you know if i find anything that corrects it.