I run 40 and 42 in the Dmax and van when loaded. I go straight down to 28psi when I hit the dirt. The rule of thumb is drop pressure 25% and speed at least 25% as a starting point on corro /rocky ground. Staun deflators are a quick way if youve got lots of tyres to deflate.
Most 4wd's when loaded are around the 40psi mark, or higher, so thats why you hear the 28 - 30psi mentioned alot.
When off road, the tyres side wall also act as a shocky, cushioning vibration to the vehicles suspension. The larger the side walls, the greater the cushioning, and they also dont have to work as hard (heat and fatigue) as the lower profile tyres. Putting 17 and 18 inch rims on off road vehicles isnt doing any favours. Lower profile tyres dont have as much side wall height, they dont ballon as much is sand or mould over rocky terrain(the moulding helps avoid cutting/piercing).