Do you feel it in the seat? Rear area
Do you feel it in the steering wheel?

Does it get worse accel or decel?? Driveshaft/Rear Axle

Try jacking it up, placing stands under the frame, and place it in drive and see if it still vibrates, if no vibration come to a complete stop and put it into 4wd.

To check your u-joints grab the driveshaft near the joint and push and pull parallel with the joint. If you see shiny spots or even a ring of rusty powder around the center of the joint where the cap seals are then its a good indicator of improper lubrication and possible need of replacement.

If the vibration is from 45+ it is usually wheel/tire related...