Range Rover had a real problem with exact same symptom and it was the steering damper. I experienced it first hand. We balanced tires no loose steering parts. Replaced the steering damper and all was fine. Inspecting the damper on the bench it appeared fine but replaceing it with another solved the problem.

Dont make the mistake i did....by jacking the car up...this puts tension on the tie rods and you wont be able to tell.
Leave the car flat as is, reach under and grab the tie rods there should be no play at all.
Also look carefuly at the front shocks. is the driver side leaking?
Measure the front fenders from the ground to the top of the wheel arch. Is it the same measurement? If there is a significant differance you got a problem.
Bounce the front up and down. with your knee. The front should be very firm no bounce at all......
A bad shock will cause a very violent shake regardless of wheel balance. Its not uncommon for a gas shock to burst at one end lifting the front on one side or pulling it down.
Just try to get that sucker out without a cut off wheel if it s pushing up!
If the gas /oil escapes from the top side of the shock the bottom pressure will push up with enough force so as to lift the car on one side and you cannot compress the shock to pull it out!

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2003 SLK 230 (85000)
1999 E300 Turbo Diesel (388,000)
1991 350SD Turbo Diesel (185,000)

I pulled the left side guide rod, bushing, upper, lower ball joints, and tie rods this afternoon. It took a whopping 30 mins.

Inspection reveals a bad guide rod bushing and a worn lower ball joint. Everything else is nice and snug.
I had replaced the rt side guide rod bushing about three yrs ago and I ordered two at that time so, I have that part.

Now, to get that ball joint out...

I love working on these cars! I hope she goes back together as smooth.

Isn't it nice not having to work on cars that have spent their entire life driving on salted roads that do nothing but rust them out

Its a pleasure to do brakes, service stuff, etc. when the bolts TURN!

No more shake at 55. See:

http://www.peachparts.com/shopforum/showthread.php?t=142212&page=5