2003 Ford Explorer Lug Bolts breaking on rear wheels

Hello,

I purchased a 2003 Ford Explorer Eddie Bauer 4.6L brand new and have enjoyed my time with the suv. A couple of years ago, my car started to wobble and I noticed that 3 of the lug bolts on the back driver side wheel were gone. I had it fixed, and then the same thing happened roughly 6 months ago on the passenger side rear wheel. I then noticed yesterday that I was missing another lug bolt (not just the nut) on the rear driver side wheel. I have been back to Ford to have it fixed 3 times now, but what could be causing the studs to break and do you know of any history of problems like these with 2003 Ford Explorers?

Thanks!

Doug