Violent shaking under braking at speed above 50 mph (sorry to all the people with KPH speedometer, I'm just too lazy to convert that for you) is normally caused by bad thrust arm bushings.

New thrust arm bushing could be destroyed in no time if they were tighten with the car on a lift and the suspension was at full droop.

Agree on this point. Also, check the steering shaft joint. Mine has play so I'll be replacing that soon to chase the shudder on braking I have. I just did thrust bushings. One I did and tightened while down. The other was by a shop and it may be shot. One thing at a time :)

Ok, when you are referring to the steering shaft joint, or you referring to item number 2 in the diagram below?

http://www.realoem.com/bmw/diagrams/y/f/110.png

Yes, number 2. Mine has clicking and you could feel it when sitting at idle and rocking the steering wheel side to side. I don't know how much of that contributes to the shudder under braking, but it needs fixing, so I'll see if it even puts a dent in the vibrations. Past that I'm back to inspecting the thrust bushings, ball joints and tie rods. I won't do the stock bushings any more, but I'll go with these:

BMW E53 X5 N62 4.4L Suspension Bushings - PFF5-901x2 - Performance Polyurethane Front Control Arm Bushing Set - ES#2649997

Yeah, I am debating on going with the Powerflex or the Meyle HD bushings myself. Usually go with Poly bushings on my past M cars, but not sure how much it would help or hurt in this application.

I'm running the staggered 20" setup for the summers and I think they are hard on the thrust bushings. I'm sure that poly on the thrust bushings will mean the next weakest link goes quicker too.

bump

I have not done anything yet, I am taking it to a friend who is a master tech at BMW to get the transmission and transfer case fluid changed on Saturday. I'm going to ask him to take a drive and inspect underneath to see if he can find anything. Will probably go ahead and change my thrust bushings no matter what though. I will be sure and update when I do that.

Just FYI there are reinforced "protection package" bushings from BMW now which should last longer than the Meyle HD I would think, based on the reinforcement design.

Are you talking about these? BMW E53 X5 N62 4.8L Suspension Bushings - 31129069820 - Protection Package Front Control Arm Bushings - Pair - ES#49337

They are listed on ECS as being for the "rear" control arm but I do not think that is correct. These look like replacements for the thrust arm.

http://c1552172.r72.cf0.rackcdn.com/414194_x800.jpg