I hear poping when I turn sharply or go over rough terrain. What is this?

do you just hear the popping?
because my explorers steering actually pops.
the pop might be a worn out ball and pinion

Yes I only hear poping when turning or going over rough terrain.

What year is this explorer? The last body style (2001 and before I believe) were known as balljoint eaters. I would check the balljoints first, and check the stabilizer links while I had it off the ground.

I have heard this noise before too. I believe Explorers are "supposed" to pop when you turn the wheel all the way when turning. Not sure exactly which parts cause the noise, but basically don't turn the wheel all the way and you will be fine.* Also holding the wheel all the way for too long can apparently damage the power steering pump. Hope this helps.

*Just read yotermanic (http://www.automotiveforums.com/vbulletin/member.php?u=287224)'s post. Maybe I should check my balljoints...