BIG Front End Vibration..

[Dudewanarace]

First off:
2001 X5 4.4i Sport Package
99,980 Miles

I have an odd vibration that starts up only after about 5-10 min of driving. It feels like a wheel is going to fall off. With both hands on the wheel you can't make it stop. Shakes to bad to go much more than 40 mph. Seems to be worse under engine braking, or weight transfer to the front. If you apply the brakes, that seems to stop the shaking. Untill you let go of them. Here are some of the things I have done:

1. Had a wheel alignment/balance done.
2. Jacked up and checked for wheel bearing play, didn't have any. No noise from them either.
3. Pushed and pulled all the sway bar links/bushings. All seems fine.
4. Front axles seem fine, all the boots are in good shape.
5. Rotors seem fine, light or heavy braking actually make it better.
6. Lugs on wheels are all tight.
7. In the snow you can goose it and all 4 wheels spin, so I don't think the transfer case has a problem.

So, I'm stuck. Anybody have any ideas?

Could I have a front differential problem? If so, how do I check it?

Thanks in advance. Sorry I only take from xoutpost, maybe someday I can provide help to others.