Do the rear calipers use the same pins as the front ones? I have heard, and seen on my front ones, that the rubber around the caliper slide bolts expands over time and stops the sliding of the caliper. I greased mine and they improved but I need to change them eventually. Morris 4x4 has them but you can get them locally as well. North Coast Wrangler (user name) on AJOR, aka Nick, works at Mansfield and he checked for me so they can be had in the SE Qld area.
Here is a LOST post on this issue:
viewtopic.php?f=5&t=53988&hilit=caliper+slide