I believe it is STILL a balance problem.
Does your shop have a dynamic balancer (i.e. the type that spins the wheel/tire?).
Balancing is the cheapest route here, and I would visit a dedicaed wheel/tire shop and have yours dynamically balanced.
Also, how worn are your tires? If you have an internal cord/tread separation problem, it will manifest itself as shimmy/vibration.
Are your tires stock size/height width. If not, you could be "out of spec"...meaning the tires are outside of the parameters that your suspension is designed for. This shows itself around that speed range as well.
Does it shimmy all the time at that speed range or just when cornering. If just when turning the wheel, then it may be worn control arm bushings.
Those are just some areas to look.
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