When the truck is under acceleration there is no vibration what so ever. However, when it is coasting I get a pretty bad vibration. I'm thinking differential or driveshaft. Hopefully it is not the transmission
My car has 203000 miles.
My car has an automatic transmission.
Check the universal joints, their u-bolts, and the nuts holding the driveshaft flange on the differential. Engine torque while accelerating or maintaining speed can keep them in a "balanced" position that changes when the torque is removed. Loose joints or bolts allow them to wobble, setting up a harmonic in the driveshaft.
I would recommend having the car's shaking symptoms inspected by a certified technician, such as one from YourMechanic, to diagnose this problem, get an accurate assessment of damage and cost estimate for repairs. YourMechanic can dispatch a mobile, professional mechanic to your location for this repair.
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