Tire balance--did you lose a weight?
Belt separated--did a steel belt in one of the tires go cattywampus? (my Dad always used that word--don't know if it's real!!)
Air pressure--is one or more tires low (for as big as it was my 560 SEL was very sensitive to air pressure)
Rotation--sometimes failure to rotate tires will cause sipe wear that suddenly manifests as if it were the Mother of All Earthquakes. (especially true with the 31's on my old Wrangler, scary!!)

Suspension--tie rod ends, ball joints, etc, etc.

Did the problem arise suddenly or has it been building? Check the easy first, air pressure. then balance. Good tire guy can sometimes spot a separated belt from the next county. (good tire guys are normally found at good tire shops, not usually in the automotive department of department stores--just my observation)

Good luck!!

__________________
Earl McLain
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)