Usually people keep the ARC as long as it functions. The ARC Explorers generally came with softer springs to begin with. My 99 isn't a good barometer because I built it for mail delivery. The front shocks are Reflex, which are a stiffer shock, but will last longer for me. My back end has stiffer springs because it wasn't an ARC truck, and is a 4WD. So it is stiffer than my Mountaineer, which got softer OEM springs.
I do believe that the ARC shocks are a good riding shock, but i have only ridden in one OEM ARC truck, and that was a 95 Explorer with 22,000 miles. It was a very nice riding 4WD truck, my friend and I test drove it in 1998.
If you have no leaks back there I would try to buy new ARC shocks. I don't know how to find that special eBay seller, but surely they are out there still. Ford sells them new for about $95 each, I bought my first pair there. Good luck,