Have you jacked a wheel up (any corner) and noticed that the wheel spins for half a turn or so? This would indicate that the diff. lock is engaged and jacking the wheel up releases the stored up torque
Generally, it is a linkage issue, primarily on the selector mounted on top of the gearbox being seized up.
A good cleaning and using some synthetic grease mostly cures the issue.
(I have found all sorts of goop in the mechanism, spilled soda/coffee, leaking sunroof etc.)
Hope this helps.
Les Parker
Tech. Support and Parts Specialist
Rovers North Inc.