MorganS:
Ok, that diagram helps. I don't really understand what the middle connection is doing for the system. That's another proportional valve and when you activate the relays, it must be off?So is the existing controller a proportional controller? That means it's already producing a PWM or analog drive suitable for the solenoids? If you still want it involved in the final Arduino-controlled system then you need to find out what it is doing electrically.
You didn't answer the question about how you expect PWM to mix the two controls.
Ignore the middle relay.. it's an old drawing.
Since using the relays to swap signals locks the transmission, I was assuming I could pwm to have them partially lock.. but I was assuming wrong.
The solenoids are current controlled, so I need to intercept the signals and then alter the current fed to them to get my desired result.