This thread helped me.
http://m.clublexus.com/forums/showthread.php?t=616815&styleid=13
Take your time and read the thread in it's entirety. I had huge wobble problems on my old 200. Had the car WIM'd, new tyres, new alloys, new lower ball joints, new lower front control arm bushes, new callipers, discs, pads..... I could not get rid of the wheel wobble completely. In the end, new ball joints and geo at WIM got it sorted.
I learnt that it's prob not one thing causing your wobble it's a combination of factors. For example with mine it was slightly buckled rims, worn ball joints and geometry out. Those things on their own May not cause a wobble but a combination of all 3 and bang, steering wheel wobble.
Oh and Lexus were pretty useless at diagnosing my wobble and it's as tho they don't really have time for it. My advice is get in touch with WIM again. Explain your problem in detail and tell them you want to get to the bottom of it regardless of time etc.... They were great with me, trying different things, sending me on a test drive, not happy come back and they tried something different etc....
Good luck, I know how frustrating that wobble can be. All I'll say us your lucky it's at 75/80mph, mine was at 60-70mph so right in every day driving sweet spot on the motorway