Make sure the lugs are correct and tight. Check the wheel bearings. Check for a loose shock. I doubt if it's the brakes. Most likely not the wheel bearings. A loose shock will thump but shouldn't speed up. Since you replace your tires I don't think you have something stuck in one of them. How about the center bearing on the driveshaft? I'm just throwing the possibilities out there and it's up to you to check them out.

Les