One thing you can do yourself it jack up the rear end and make sure it's very stable. Then put the car in top gear look at the rims for wobble or excessive runout. If the car doesn't shake, give it some revs to get it to 100-120kph. If you don't hear/see a vibration in the back, swap the front/rear wheels and retest.
If you do get a vibration, that will help you pinpoint which wheel it is.
If you don't get anything, especially since you feel it in the steering wheel, I would suspect your suspension.
The FCABs and balljoints can cause your issue. Watch these videos to see how to check for it.
Some other questions...
Have you had an alignment done recently?
How old are your tires? Do they have any unusual wear?
Are your rims stock offset/width?