Sorry I wasn't clear in my description.
As I understand it, the tandem pump has two separate circuits, one for the power steering and one for the ASD.
Each circuit has its own reservoir, fluid and lines.
I don't believe they normally interact, unless there is an internal breech in the tandem pump.
What I meant for you to try was to loop the high output ASD line at the tandem pump back to the low pressure ASD return line at the tandem pump.
Effectively cutting the hydraulic fluid to the back half of the ASD circuit controlling the differential, but still keeping the ASD portion of the tandem pump lubricated and operating under a no load condition.
You might need a temporary reservoir to bleed the bypass loop.
Same idea as mini tank under the hood to bypass the fuel tank.
The tandem pump appears to be part number 126 460 15 09 which is NLA, but it has been replaced by part number 126 460 21 09.
It also appears NLA.
A leak in the steering box wil definitely cause steering difficulty and perhaps may affect the tandem pump's ability to pressurize the ASD circuit.
Here's a good pictorial on rebuilding a tandem pump:
http://www.peachparts.com/shopforum/diesel-discussion/179784-tandem-power-steering-pump-rebuild.html