I used to run 3-5 PSI less in the rear tires of my pickup truck as well.

For my Commander i run them all the same pressure. I'm not sure what the exact weight distribution is on these vehicles, but it looks to be pretty close to even.

My "standard" procedure is to run them all the same unless i see signs that pressure should be reduced, ie: the center tread of the rear tires wearing more, or non-uniform tread contact patch.