I would agree with Wes that many vibrations can be traced back to the driveshaft. However, I have been trying to pin down the slight vibration that occurs on my truck from 55-60 mph, or around that 1500-1700 rpm range (3.55 gears) for over a year. My first thought was driveline, so checked all the u-joints, carrier bearing, etc. Checked the exhaust piping too. I can feel the vibration in the floor under my feet, so seems to come from up front. Checked wheel bearing and ball joints-all okay. When the vibration happens, I can turn of OD and kick the rpm's up and it goes away. That tells me it is somehow engine related. I began to suspect my fan clutch, and had a new one on hand, so installed it. Nope-no change. I recently saw a comment on a thread on some other forum that the vacuum pump can cause an rpm related vibration. Anyway, I am still searching for the cause, but may I have provided the OP with some more ideas about what to check.
