i've had a bent axle once, and it shook the entire time, not just at certain speeds. However, mine was bent "alot". when I jacked mine up and spun the wheel without it being in gear you could see it being out, either way its worth a check. Another thing you could check is make sure you lugs for the bed aren't missing from under that side of the bed, at a certain speed if the nuts ar emissing the bed might rattle and vibrate so badly that it feels like what your having issues with? (big grasp)