Ok been reading through the steering issue posts and can't find one that sounds like the problem I've run across.

Here's everything I know about this truck:
2000 dakota 4x4 quad cab 4.7L
power steering is a little difficult to turn.
Pump has been replaced 6 mos ago
No leaking in the R&P...
Hoses are a little greasy looking but no easily apparent leaks
Tried greasing up the intermediate shaft u-joints, no change.
Sounds like the pump is working hard, gets a little easier when engine revs up.
I'm looking at a p/s pressure switch... What exactly does this do? Would a bad switch cause this problem?
I'm getting new pressure and return lines installed right now.
Also going to flush the power steering system... Hopefully this will fix the problem but posting just in case someone else is having this problem. Any ideas/solutions I haven't thought of?

 

The PS pressure switch kicks the idle up when the pressure is high in the system so you don't stall the engine. Is this just an at idle thing or all the time?

I'd check those u-joints. Disconnect them and see how well they move. They can cause hard steering. I'm not sure how you tried to grease them.

Check the tire pressure too. It can make a difference.