The rear end on my Mountaineer (97) was sagging pretty badly on the left hand side in the rear when I got it (with a LOT of miles on it). Ford quoted me ~400 USD to fix that one side. I ended up taking it to a local spring shop (which came highly recommended) and for ~450 they did both of the rear springs and put in what I consider to be a more reasonable set of springs. Compared to the four leaf set that was under there when I got it the new six leaf set is much sturdier. They would have used a 5 leaf set but when I told them that I planned on towing with it they set it up this way. The ride is a bit harsher, but now it at least does not sag. It rides more like a truck but I do not object to that at all.

When I was looking into possible replacements I recall reading that in the case of the 2-door Explorers of your vintage the rear springs are not as strong as the ones on the four doors. A lot of people seem to put the 4-doors' springs on and have excellent results (edit: looks like jhrsmail posted this same advice as I was typing )

My opinion, take it to a good local spring shop.