Are you looking for a manual transmission or an automatic? I really like having the rear differential lock as open diffs suck. The Pro-4X is just a rebadged Off Road with no real difference other than the interior color, and embroidered carpet floor mats if you went for the floor mat option. If you can find a 2005+ Off Road trim, I would go for that. I was looking at a used Off Road Xterra before I purchased my truck, but the dealer was asking way too much for what it was worth. It made more sense for me to factory order a 2011 with everything I wanted instead of just dealing with what I could find.
The Off Road trim comes with skid plates, the rear diff lock, bigger tires, etc. and is totally worth the extra money. If you are looking at an automatic, you can still probably find some 2010's at heavily discounted prices. Otherwise, picking up a slightly used one is still pretty easy. Finding a manual transmission however, is not an easy task. Look around though, it may be worth driving a few hundred miles to get exactly what you want. I would honestly test drive a few examples of whatever vehicle you are considering. One Xterra could drive completely different than another, especially when comparing used vehicles.
I have taken my truck up some extremely rocky, washed out, muddy, slightly snowy and steep trails/roads so far and have only used 4WD once, even though I have only owned it for 2 weeks. I have yet to actually engage the diff lock but am looking forward to putting it to use. From my very limited experience so far with the Xterra, I am very pleased with my purchase and look forward to testing my/and the trucks limits. I do however hate the traction control, and turn it off anytime I get off paved road.
I would recommend getting rid of the stock BF Goodrich Rugged Trail tires however, they are complete junk. I am getting new tires tomorrow afternoon and plan to sell my stock tires with 500 or so miles for some decent money.
