Ben, I had a vibration problem that drove me crazy for almost 2 years. I once felt the same vibration in an EL my father was test driving. It ended up being the tail shaft.
It was not normal tail shaft vibration (I changed 3 sets of universal joints during the process), rather it would give a slight vibration at only certain speed(usually 80-85km/h), reguardless of rpm. And it was worse with more weight in the car.
Anyway, to cut a long story short, check out your tail shaft. Should not be more then $20 from a wrecker.
CAUTION: Slippery When BOOSTING!