Coolent Type??
 

Just got home and noticed the radiator was bone dry. Don't know where it went.

I bought the truck used so not sure of the type. Can the old green and the newer orange be mixed?

I'm hoping that this isn't a head gasket as I hear it's a common problem for the 4.0's. I have 95,000 miles. Is there a normal mileage when it goes out?

The orange coolant is dexcool which is for general motors vehicles. Ford has a gold extended life coolant or a basic EG green coolant. check the resevoir to see the color or if its dry look on the bottom of the radiator for the drain and turn it till a little comes out to see what comes out. When you find out what is in the system top of the level and have a pressure test done to find your leak or leaks, I do this regularly at work on many vehicles. It could anything from resevoir leaking to head gasket or the seams on radiator.