shocking experience
Theres quite a few options for shocks. What you choose should probably be based on what kind of ride your looking for (smooth and comfortable / firm and sporty) and of course, price.
Standard OE replacements are available from Monroe and Gabriel at auto parts stores. The Monroe Sensatracs are a good shock for the price and offer a good compromise of a decent ride and performance. The Monroe Reflex are their top of the line and are probably what you want for an easy-to-get sporty shock.
Theres a few other performance options from Rancho, Eldebrock, Tokico, and Bilstein. These shocks tend to ride firmer and can be somewhat more expensive, but for all-around control they are a great upgrade. You can get Rancho RS5000's at Sears, Bilsteins and Eldebrocks can usually be sourced through parts stores that sell their product lines, and Tokicos through performance shops. Online stores also sell most of these products at some good discount prices.
If you want a soft ride and aren't too concerned with performance, you can just go with one of the Monroe options from a local parts store and be done with it. If you didn't mind a firm ride and feeling some bumps, the Eldebrock IAS or Bilsteins are some of the best performing shocks there are for the Explorer.