Are your tires the factory spec size? In some cases, this can cause the problem. I put oversized tires (onto stock rims) on another car I owned, only to be plagued by an annoying shimmy at around 35-45 mph. Tires wore out extremely fast too! Replaced them with the factory width, and the problem disappeared!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car