If your vehicle is showing drivetrain vibration that gets noticeably worse when coasting, here are the most likely causes and the parts you'll need to fix each one.
U-joints allow the driveshaft to flex with the suspension. When the needle bearings inside dry out or bind, they create a high-frequency vibration that often peaks when you let off the gas and the driveline torque reverses.
On vehicles with a two-piece driveshaft, the rubber cushion holding the middle bearing degrades over time. When coasting, the unloaded driveshaft can whip or sag, causing a noticeable shudder under the floorboards.
Broken or collapsed mounts allow the engine and transmission to shift out of alignment. During deceleration, the powertrain tilts forward, altering the driveshaft operating angle and transmitting vibrations directly into the cabin.
Installing a lift kit or altering suspension height without correcting the pinion angle causes the U-joints to operate at unequal speeds. This creates a harmonic vibration that is highly sensitive to throttle lift-off and coasting.
Bearings inside the differential support the gears. When they wear out, the gears mesh poorly when coasting (which loads the back side of the gear teeth), causing a deep vibration that is usually accompanied by a whining or howling noise.