If you can feel it when you kick the tyre I doubt its the subframe, the bolts for the subframe are huge and if they were loose enough to move when you kicked the tyre I think your engine wuld have fallen out by now.
I reckon its one of two things,
1) droplinks, mainly because they are right behind the wheels and are one of the only things you can feel knocking through your hand or foot while the car is on the ground. If you've not had them changed, I 'd recommend getting them done, they can make a huge improvment in driver 'feel'. They are also relatively cheap compared to other repairs on the Supra.
2) Track rod ends, I think these only because you've mentioned the steering shake on motorways, sometimes a telltale sign of wear. Easy to diagnose, jack they front wheels up and rock the steering from the wheels, if you feel play or slight knock, my money is on them.
Good luck mate.