Could be a sign of a failing fan clutch. It may not be drawing enough air thru the condensor at idle speed to cool the refrigerant sufficiently. When you rev it up slightly after it's been idling for a while, do you hear the characteristic "WOOOOOO" of the fan?
When the engine is stopped, see how much resistance is felt turning the fan blade. You should feel some drag.
When the engine is idling, you should feel a decent amount of air being pulled by the fan. If you can do it, at idle, the fan should draw enough air to hold a shop towel to the condensor. Raise the engine RPM, and it should suck it right tight to it.
Lots of times, the low side pressure isn't totally indicative of the state of charge of the system. I had a Durango mislead me that way...Low side read perfect, but it was 1.5 pounds low when I had the system totally evacuated and recharged.
If the low side is low to normal, but the high side is high, it could either be a fan problem like I mentioned earlier, or an expansion valve (H-valve) that has a problem. I'm not sure if the Explorers use an orifice tube or not...Typically that was a GM thing, but a clogged orifice tube can also lead to low cooling at low RPMs, due to restriction in the system that the pressures overcome when it's turning faster.
