I have got the shakes (yes I searched)

after 25 mph my jeep starts to shake, I let off the gas it goes away. Its not too violent, just bad enough to make the rearview mirror blurry. It just came out of nowhere. It is not a u=joint rumble as it makes no noise. Can someone please diagnose this----Kyle

update--------pulled front shaft and noise still there.............threw in neutral while moving and smooth again so no shake under load. Either bad u-joint in rear d-shaft or t-case problem------Kyle