I have just been dealing with the exact issue. In my case unfelt a slight wobble when not braking and it got much worse under light to medium braking.

Just an FYI- most tire shops that "balance" your wheels do it incorrectly. I had 3 shops rebalance and say the last was off. Trust me on this: find a place with a hunter gsp9700 and have them do a road force balance. I would highly recommend going to a local shop or someone that clearly is going to put in the time your wheels Probably need.

If the doesn't fix the problem completely, which my hunch is it won't, it's very very likely that it is the rotors....