Maybe an upper shock bushing? Just a guess. You can feel the vibration through the body and it can sound/feels like a ball joint or something that's loose lower in the suspension. When you jack the car up, you load the bushing, so it's hard to detect. You might be able to feel it if you put your hand on the top of the shock and have someone lift the fender up and down as hard as they can.

__________________
Reid
Past ('59 220Sb, 2-'59 190's, '92 300D 2.5T)
Current
'04 E320 4Matic Wagon
'00 E320 4Matic Wagon