Ian,
Without a cross axle diff lock, your rear axle does not drive both rear sides, especially off road. Most Rockys (Fourtraks) are fitted with open diffs front and rear, which means if you lose traction on one rear wheel, in 2wd, without momentum, you will stop, one wheel stationary, one wheel spinning. The diff transfers all the drive to the wheel with the least resistance. In 4wd, you have the same situation when you have 2 wheels with lost traction, usually diagonally opposite wheels. That is why, if you go diagonally across a ditch, slowly, you will usually end up stuck, with two diagonally opposite wheels spinning madly, and the two others sitting still. Dispite popular belief, 4wd's dont go everywhere.
To get constant drive to both rear wheels, a cross axle diff lock needs to be fitted, which are not readily available for our cars. For the ultimate in traction, fit cross axle diff locks to both front and rear diffs. That way, you must have all 4 wheels lose traction to be stopped.

Hope this helps explain your problems
Cheers
David
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