Also when shifting back to 2WD just backing off the throttle so the engine pulls a stronger vacuum may be needed. Where you reverse the rig gets a few cycles of throttle/ vacuum more so than any action on the driveline.
I find the system can take quite a few second and much longer when cold out to release the front drive.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.