Ahh, isn't it great when you reach the point of a boatload of maintenance?
Sounds like you should be freshening everything up pretty nicely though, so with luck that will help smooth out your idle.
As for the sliders vs. skids thing, I agree with what others have said. Think of it this way: the Xterra has a pretty short wheelbase, which means the odds of coming down on the undercarriage is already fairly minimal, especially with the lift and tire size you have. If I recall correctly, everything vital is also tucked up above the frame rails on the first gens too. So that means body contact is more likely, so sliders are the way to go. As far as which ones, I highly suggest the White Knuckle sliders. Trust me, the HREW is more than strong enough for your uses. DOM tubing is for really heavy vehicles or rock crawlers:
http://www.savageoffroad.biz/DOM-vs-HREW_ep_42.html To be honest, what matters most is the steel, not the process by which it was turned into tubing. If it's good steel, it will be strong no matter the process.
I also prefer the design of the White Knuckle sliders. They are really well thought out and the finish is excellent. I love that the square tubing is tapered at the ends to make sure everything is covered from damage. I also like their rear kickout more. It flows better and looks cleaner in my opinion.