five things it could be: Cheapest first!
1, Wheel balance weight has dropped off, never assume that just because a new tyre was fitted and balanced then all is ok, they can fall off if they were not stuck on properly, in particular stick on type on the inside of the wheel... It gets really dirty and the tyre fitter may have notcleaned the area properly!
2, Are you sure your wheel bolts are tight? Your wheel could be wobbling if they have worked loose.... Any wheel once removed should have the bolt torques checked after 50 miles or so... Check them right away!
3, A wheel that is buckled and out of shape. Again, don't automatically assume they are fine, its easy to check visually by jacking the rear of the car on each side, releasing the hand brake and spinning the tyre/wheel fast while watching the tread to see if it raises or dips... Now remove the wheel and check the inside visually. Any flatness instead of perfect roundness will be apparent.
4,Wheel bearing grumbling. While you are doing check number 3 If you here a grumbling noise while spinning the wheel then your wheel bearing could be on its way out. Grab the wheel top and bottom, side to side and try wiggling it as if you are trying to remove it off the hub while still bolted on... Any excessive movement will confirm this.
I doubt it could be anything more than those items although driveshafts are a possibility with it being a JTD... Even in standard form they are on the limit of engineering performance. CV joints too but you would get a clicking on 3/4 to full lock... Driveshafts can be physically checked when the front end is suspended on a proper ramp.