[guyryan100]

Mine was old tires. I've read some interesting troubleshooting ideas on this site, including narrowing it down to front or rear based on whether the rearview mirror in the cab shakes, indicating front end problem, or you feel it in the seat, suggesting its the back end. Also whether it only shakes when you are accelerating which suggests an engine-related issue. I jacked up one rear wheel as a test and found no vibration with the rear wheels spinning, which eliminated transmission and driveshaft issues.