I know squat on the CAD... but found this:
http://www.ifsja.org/forums/vb/showthread.php?t=132957
If there is no Vac. hose going to the front cover, and you have no front drive shaft on it now, just get under there and turn the yoke on the front diff, if you can turn it, it is disengaged, if you can't the PO has done something to keep in engaged...........Might want to open it up and check to see what that was. In any event if you can turn it, hook up a hose to the front diff cover, and run it to manifold vac. start up the truck and then try to turn the front yoke, if it is working correctly you won't be able to turn it very far before it locks. I always thought that Disco front end was a retarded setup, while you get rid of the drag of spinning the diff, you still are spinning the axles and wear and tear on the axle joints ect ect. I would put the hubs on, and leave the vac line hooked up, or lock the front end by some other means.
IIRC, there was a guy here that took the vac motor a fork off his axle, and used a hose clamp on the shaft to keep the collar in the engaged position. But I'm running on 1/2 a cup o coffee, so could be mistaken...
I have never messed with one, but from what I understand, you take the cover where the vac motor is off and it will have a shift fork on it that gets removed. Inside, the axle will be in 2 pieces, with a collar that slides across the break to lock them together. Engage the collar and put a hose clamp on the shaft behind it to keep it from disengaging.