My 1992 4-runner 22 re has ceased to be a 4x4 suddenly. I found out the hard way, slimy mud in a remote location.
I've gone over the whole system as best as I can. I know that the Transfer case is working properly because as I was spinning my rear wheels in the mud and not moving at all the front driveshaft was turning but no action at the front wheels
As far as I can tell vacuum is what activates the differential to make 4x4 happen. I tried plumbing vacuum directly to the hoses that run to the differential. None of these lines leak and the vacuum is strong and consistent. No 4x4 still.
Any trouble shooting hints for this whole system? Any cheats to bypass parts of the system to see what's failing?
My tests are not very systematic as I don't really understand the system so I may have missed something. If I haven't I'm at the conclusion that the front diff is failing somehow. It has never made any noise or been stressed or abused since I've had the vehicle and it did work a few months ago.

thanks for reading, help?

look here