I'm a bit stumped why your resonance issues are so bad and at such a low speed?

Maybe it's some issue with your particular type of stepper driver? Some drivers require a large PSU cap on their power terminals, I have some old driver modules here that don't say anything on the module but it is mentioned in the datasheet; they need 2200uF adding across the power terminals close to the module. Which is pretty good advice for most stepper drivers anyway if they need to do higher speeds. Your large motors may even need 4700uF. Since a 2200uF 63v cap costs about $2.50 it's probably worth trying as a test before you buy new drivers.

Also are you trying to run the motors continuously at that particular resonant speed? It's best to just accelerate through that speed reasonably fast. Even a well set up system can tend to break out in resonance if you run it at just the right (wrong?) speed for any length of time, with little to no load. Just another thought.