CV joint? I know this would be the middle of the truck, but vibrations can play funny tricks. Needle point tip on grease gun to get it.... almost have to take the front driveshaft off.
Cv is static in 2wd. Just sayin.
But if front u-joints werent checked they can cause vibrating too. As they arent manual locking hubs. Too much speculating if its a 2wd.