I haven't replaced freeze plugs on a 4.0, but have done the job on V8's. They should be on the sides of the block in the motor mount area, you usually have to undo the mount, jack the engine up, and remove the half of the mount that bolts to the block. There isn't much room to work, but sometimes you can drill, put in a sheet metal screw, and pop it out with a craw hammer. Other times you can drive a screwdriver through it if it's rusty enough and pop it out.......not one of my favorite jobs.LOL
1999 Ranger Extended Cab, 4x4, 4.0, automatic.