Wheel balance, bad tire, steering rack bushings, inner/outer tie rod ends or combination of the above. Rotate your tires and see if that helps. If it does, rebalance wheels. Check rack bushings by watching rack as a helper moves steering wheel back and forth with the engine running. If rack moves horizontally at all, bushings are shot. Check TRE's for play. Put front on stands and see if there is play when you push/pull wheels at 3 and 9 o'clock. Also check at noon and six for ball joints, etc.
I had a bad wobble in wheel and shifters above about 60 mph and I did new tires/wheel balance (needed new tires anyway), rack bushings, shocks (needed it anyway), and alignment. Each thing helped a little, but tires and rack bushings the most. Now it is rock steady up to 95mph which is faster than I want to drive the thing.