Diff Lock Seized

My diff lock control lever will select high and low gear, but will not travel across to lock the diff.

Any idea how to free?

My helpful garage disconnected the linkages, but explained the problem was at the top? and they were disinclined to remove all the interior trim to gain access.

Its not really that dificult to gain access, you should be able to get the trim away with ease. the only snag is the rubber membrain that is pop riveted over the gearbox. Drill the rivets to remove and the shaft you need will be obvious.