Bottom Line Up Front (BLUF): I believe the problem is a totally bald passenger side rear tire.

Okay, you all can rag on me and give me a hard time. I believe I have identified the problem. Took it out and inspected under carriage, wheel wells, wheels, lug nuts, and tires. Did not see anything remarkable although one of the tires had ever so slight movement in lug nuts when I tightened three out of the five. This was rear driver side tire. However, upon closer inspection of the other rear tire, I found it to be as bald as a baboon's you know what in the middle. My pressure dropped in all four tires, as well, probably as a result of the drastic (30 degree temperature change we experienced in the last 24 to 36 hours). I am hoping that the real issue was / is the balded passenger side rear tire. Although, I am still skeptical since this problem seems to have arisen quite suddenly. In any event, on my way back to where I am staying, after washing it thoroughly (thanks Ozarkdude) and having tightened all the lug nuts on all four tires, I did not experience the earth-shaking, Richter-scale vibration that I had up to approximately 65 mph. that is not to say there is not still an issue as there still seems to be latent, uncharacteristic vibration and I did not open it up on the interstate to beyond 65 as I had done earlier.

The other three tires are fair to good in terms of tread / tread wear, but the one is unacceptably bad. I am thinking to have it changed tomorrow before I get on the road. I will probably buy a Michelin and then use it as a spare, take the other four, 1 year-old Michelins off the 400E and try to keep my 300E operational a little longer.

However, if the problem is still not solved, then I will still be at a loss as to what it could be.

One other clue. I am usually able to put 320 to 330 miles on a tank before the gas light comes on. Tonight the gas light started to flicker at approximately 300 miles! Very unusual; in fact, it never happened before. The car has religiously gotten 20 miles to the gallon in the winter and between 23 and 26 in the summer. When that happened I mentally kicked myself in the rear for not having checked the tire pressure before I drove the 200 mile trip on Thursday up this way. Could the bad tire / low tire pressure really knock off the 20 additional miles I am used to getting on a full tank? Again, if I did not know this car so well, i would not make much out of the 20 mile difference, but the gas light has always been spot on in terms of gallons / miles left. I've driven 50 miles without running out of gas with the light on full-time. After that, I put 17.5 gallons in, which tells me i had about 10 miles or .5 gallons left (again when I was getting a minimum of 20 mpg) 18X20 is 360.

Thanks again to all who replied and made suggestions, especially Ozarkdude.

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CJTucker