2000 and newer for expiditions for s.a.r.

I'm in search and rescue and our unit is one of the last units in the western united states that operates soley off of donations from the community for our operations. Unfortunately our mid 90's jeep cherokees are wearing out and we are slowly inheriting ford expiditions from the sheriffs department. Our problem is the expeditions are terrible in sand or mud. Can anyone help me with possible upgrades to allow these expeditions to perform in the areas we need them to. These expeditions are all stock. We have a limited budget but our equipment does take priority due to our mission but please keep in mind if we could afford bradley's or tanks we would have them already.   any help is greatly appreciated by our unit and those we help.