It's probably tires and/or balance. If you get a resonant vibration that changes with rotational speed, then it's most likely a rotating object out of balance. Just replaced the front tires on my '89 300E and the vibration went with the old tires. May not need alignment unless there is abnormally distributed wear across the tread surface.

------------------
Jeff L
1987 300e
1989 300e
1987 BMW 325