OK guys,
Having been a GM alignment tech in the early 60's, and a NASCAR mechanic at another point in my life I think I can give you guys a simple solution to your problem. Keep an eye on me Gilly as I've never done this on a ML. On my ML 430 the steeering arms are angled behind the centerline of the front wheels. The tie-rods are attached to them on one end and the steering rack on the other. It APPEARS they are adjustable. If your steering wheel is offset to the left it means the LH tie-rod is too long and the RH tie-rod is too short even though the toe-in measurement is correct. If you shorten the LH tie-rod a 1/6th turn and lengthen the RH tie-rod 1/6th turn it should begin to center the steering wheel without changing the toe-in measurement. Take it in 1/6th turn increments until you're happy with the steering wheel postion. It shouldn't take much adjustment. It would then be prudent to take it to a good alignment shop to check your toe-in measurement hasn't been changed. Best to take it to a MD dealer. I suspect when you had your alignment done they didn't lock your steering wheel in the center postion when they adjusted your toe-in! Good luck!!

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David L