Trying to locate the source of an annoying low RPM vibration. The car is an '85 240 DL, B230F, M46 4spd+OD transmission. 280K miles. Issue I'm having is that the drive line vibrates badly if I attempt to accelerate from 1500 rpm or less in third and fourth. For example, if I get down to around 20 mph in 3rd gear and then goose it (instead of shifting down to 2nd) the car vibrates until I clear about 25 mph or 1500rpm on the tach. Does the same thing in 4th when I try to accelerate below 35 - same engine speed, 1500.

I may be at the bottom of the gears at those speeds but I don't thing that the car should lug/vibrate the way that it does.

The tranny and engine mounts are five years and maybe 60K miles old. The center carrier support has about 100K miles on it. As far as I know the driveshaft is aligned properly. Transmission hasn't been touched for about 10 years (other than fluid changes), since the original clutch was replaced.

Any ideas? TIA.