wheels
rotors
sticking caliper
axle
diff
any number of suspension parts

have u bumped a curb or anything recently

i had this problem when i had warped front rotors
some people say that rotors only cause vibration, but i can say from experience that my steering wheel would constantly shake at any speed below 80
it was odd, above 80 the shaking would go away almost completely.

and if i hit a bump or pothole at any normal speed the car would go into a shaking fit for a minute or two.

change your rotors cause thats the number one thing that could caues this on a normal condition prelude.

~kris