BAD VIBRATIONS

[PICKUP6772]

Hey guys. Just for shits and giggles, I drove VERY conservatively last week, and actually picked up 2-3 MPG. But in the interim, when holding steady between 45-50 MPH, I have a vibration. It's not tire balance, as I rotate, and balance them every other oil change (6,000 miles).It feels like it's coming through the floorboard. I've gotten the car to speed on an empty highway, put the trans in neutral, and it doesn't really make a difference. I'm leaning towards it possibly being the driveshaft out of balance. I have the old shaft from my old 89 CV, and I have to change the rear trans seal anyway. I was wondering if anybody has had the same problem, and what else I should be looking for. Thanks in advance.
MIKE

[gadget73]

could be bad U joints too. Does it clunk when you shift from reverse to neutral? I'd also have a look at the rear trans mount, particularly if its leaking. They turn to jello and let the trans move around.

My DS is out of balance but it vibes over 75, but then again every situation is different.

[PICKUP6772]

Gadget, Yeah, I changed the mount about 6 months ago. I had an oil leak (valve covers), that took care of that. I haven't had any clunking at all, but I'm putting joints in the CV shaft, before I install it in the Merc.
MIKE