Thank you guys for all of the help!
After scoping the network common, I found that there were 5-9V peeks of noise all over that line while the drives were running, and only fractions of a volt of noise while they were off. Clearly this was the issue.
I decided to rewire the network following, what I know to be good practice, vs following what is described in the RA Powerflex40 manual. The manual does not have the network common terminals (terminal 04) connected to network common, but instead tied directly to ground for all nodes. Instead the rs-485 standard for using a single twisted pair in shielded wire, is to connect the shield to network common at all terminals - then ground the shield at the furthest node.
Removing the PE connection from terminal 04 and replacing it with network common on the shield fixed the issue entirely. There is still noise on the line, but less than 1V at it's largest peaks, instead of 5 - 9V peaks!! I will also be adding an Isolated repeater from the PLC (master node). But for now, adding an 480uF electrolytic cap and small inductor to the PLC power terminals has cleaned up most of the noise all together. Peaks only around 100mV while the drives are running.
Thanks again for your help and your suggestions! I hope this solution helps anyone else who runs into similar issues in the future.