I used to get weights put on the inside only for cosmetic reasons...but this was for a show VW Beetle, that for the most part saw the lower end of highway speeds.
You CAN get lucky with some rim and tire combinations, but usually, the only real way to ensure that your tires and wheels are perfectly balanced is to mount the weights inside AND out.
Once you can safely rule out a wheel balance problem, if the shimmy persists, then perhaps suspension or driveshaft components could be the culprit...
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