Shaking wheels We were driving @ 55 MPH when we crossed a railroad track. We must have left the ground in that second and when we got over the railroad tracks, the truck shook profusely or rather, the tires did. This happened one other time when we hit a high spot @ a bridge approach. What happened?

Drive shaft!

loose ground some where check all the grounds for corriosion and looseness

I guess my first question is how do you know it is just the left front wheel ?
Have you checked the front shocks ?
Does it make any difference if you are applying the brakes ?
Have you moved the left front wheel to a different position ?

Start by checking the tie rod ends on both sides on the front wheels..With the truck jacked up and jack stands underneath the frame both left and right wheels should be suspended in mid air..The are two tie rod ends on each side of the vehicle, a inside and outside tie rod..Check for any excessive place on each one...
Also ckeck the balance on all four tire's... Inspect the wheels for any stress cracks that would cause them to not to balance correctly and make your vehicle shake...
Check the transmission mounts on the cross member,make sure the mount is secure to the Trans and cross member mount..Also check both u-joints on the drive shaft and any hanger bearings that may be underneath there..If it's a 4 wheel drive check both drive shaft's...Anyone of these option's could show those
symtom's you mentioned...Good luck and let me know how it works out..

1. out of balance tire because it sounds speed spacific.
2. worst of all! Loose lug nuts or possibly loose spindle nut. (you will need to lift the wheel off the ground to tell)
3. worn linkages i.e. ball joints, steering linkage, anything that would allow play in the front wheels.

You need to find if your problem is coming from front or rear first.Do a test drive where you can drive at slow speed with no traffic an a good road.At 10 to 15 mph,
notice if the steering wheel is rocking left then right.If itis your problem is in the front wheels,if it doesn't pick up speed to 15 to 20mph,notice whether your back side is rocking left and right,if it does your probable is in the rear wheels.Next is to see the wheels on a spin balancer to check for steel belt shifting in tire or bent wheels.If none of this helps you didn't mention any sounds,but check the u joints

Hi:
Since your BMW 540 series is a rear wheel drive car, a common cause of that sort of vibration is often the U joints on your drive shaft. The vibration often only shows up and certain speeds.

It isn't a particularly expensive repair, but is something to have checked out!

Has the upper strut bushing been replaced? Have you checked to see if the wheel is loose? To do so jack up the side you here the noise on and use a tire tool or piece of pipe and jack up the wheel when till it's off the ground about an inch enough to get you bar under the wheel. Next pull up on the wheel with the bar if you feel a clunk then get a second person to pull the lever up and down and see where the looseness is, if there is no looseness when prying up and down then you will have to wiggle the wheel from left to right to see if you can find the noise, a also you will have to pull on the drive axle to see if it's loose once you determine what is loose then you can tell me and I will tell you what has to be replaced.

have a tire shop balance your front tires.

crankshaft balacer going bad,can cause the crank to break if left unchecked