Check the rolling circumference of all the tyres. Any difference of more than 10mm in circumference can cause gratuitous 4wd operation (anything more than 2mm tread depth is frowned upon too).
Ignition off, all wheels off the ground, if you rotate the rear propshaft do the rear wheels turn?
If so, the 4wd clutch pack could be locked up.
Finally, try either removing the 4wd fuse, or disconnecting the 4wd clutch pack multi-plug on the nose of the rear differential.
Then roadtest, if it drives okay, then new rear diff clutch pack required.
Don't let them drive it locking up for too long, rear diff units aren't cheap when they go bang....
