M.B.DOC thanks for your reply .The trans. mount was replaced 2 years ago and has less than 1800 miles.I checked the rear subframe mounts when I installed my re-built axles in Oct. 2008 they were in good shape.Would bad mounts cause a vibration only in 4th gear?This is the only time I get the vibration is in 4th gear.I can drive at 55 mph with the vibration, and if I floor it to hit passing gear the vibration goes away,as soon as it shifts back to 4th gear the vibration comes back.