Finally found something that works well for me. I bought a kit from Timbren . Its just a big rubber spring that goes between the axle and the frame (in place of the factory bumpstop). It works awesome! It rides like normal most of the time, but on the occasional big bump or when carrying a heavy load, the rubber spring supports the weight. No more bottoming!

Here's the complication. They don't yet make a kit that really fits. The one they list for the 05/06 Xterra is the TORTUN4. I have no idea why they list it, because this kit is for spring over axle, and we have spring under. IT WILL NOT WORK! So what I did was make my own mounting bracket and used the TORTUN4 rubber spring. The rubber spring mounts to the bracket, then the bracket bolts in place of the factory bumpstop. It leaves about 3/4" space between the axle and the rubber spring, which is what Timbren specifies.

FYI, before this I tried bolt-on help springs and they were WAY too stiff. These things are awesome!