Could be a lot of things.
-worn lower control arm bushings
-Worn tie rod ends
-Worn/leaking CV joints
-Bad wheel bearings
-worn u-joints in the drive line
-stuck brake calipers
I have a steering wheel wobble that starts at 60+ MPH. Im going to be diagnosing my problem here in the next couple of weeks. I'll make a thread here soon.
I had this same issue and after a good bit of diagnosing, I replaced the lower control arm bushings with poly bushings. Problem solved!
Not saying it's definitely your issue but it's a possiblity.
LCA bushings replacement