Sounds like wheel bearings to me. If they aren't slightly loose, they might be worn to the point the need replacement. If you replace the bearings don't forget to replace the races too.

My 4.5 had the same wheel shimmy under certain conditions, to include the one you listed. I repacked and tightened the bearings, shimmy went away, at least for now. In the next few months I will probably replace the bearings, just because.

__________________
Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.