1998 Chrysler Town and Country Rear end shakes at speeds ov

Rear end shakes excessively at speeds starting around 60-65mph. I have had all wheels balanced 3 or 4 times with little improvement. I even replaced the wheels and tires with a completely different set and still the same shake occurs. My mechanic keeps thinking that balancing is needed but I'm convinced it is something besides this. This shaking started after I had some brake work done so I'm suspicious of something that happened during that process. I have since rebuilt the rear brakes thinking this might help but still the same shake. Could something be bent or warped? How could I tell. I thought about replacing the shocks on the rear as another "test" but they are around $200 a piece so didn't want to gamble that much without good reason. Any advise greatly appreciated!

Thanks,
Joey