You could just try putting a pressure gauge on the pump to see if its low on pressure, and not operating the rear diff strongly enough, or a faulty solenoid not letting the good internal pressure out into the diff.
I did a search on Google for a "AYC pump Tester" and rented one from an MLR trader for a week for £20.
Pump was actually fine, but it was one of the solenoids was staying jammed in after being operated, not letting out the Juice when asked. Its a sealed system so hard to spot whats going on inside the pumps, as the mut test just showed all was good electrically, but the ayc is a hydraulic pressure system, and grit or corroded pumps dont show up, unless you take the time to check the output end too.
Hope that help, Chris (newbie on here!)