Not engaging 4wd - '96 3.0 diesel Surf

[DMNTD]

Went for a pootle around the Waimak today and initially the 4wd was working with nil worries.
Went over a bit of a ahhh bump and now it doesn't seem to want to engage 4wd despite showing that it is on the dash.
No strange noises or anything obvious

Any clues?

[DMNTD]

Ok...now that I have opened my eyeballs and seen a similar thread I'll try some of those suggestions
However if someone does have an idea/clue please do post it up

[tweake]

jack it up and see if diff turns. could have broken cv or diff.