I have a low pitched rumble that starts at about 40 miles and continues on up to about 65 or more, Like a shake but not in the steering wheel, more like in the seat and frame.
Tire/wheel problems are at about 1/3 the frequency of driveline problems, and are usually better described as a thrumming or a shake. Could this be a rear wheel problem, perhaps a bad tire? You could rotate the tires and see if the problem moves to the front. Also look for clots of mud in a wheel, etc.
Once you've experienced both wheel and driveline problems it's fairly easy to distinguish based on the frequency, but it's awfully hard to describe to someone else. -- Jim
