Rear Suspension Shock Absorber problems of the 2003 Mazda Tribute

I own an 2003 ES 4x4; during my last two scheduled maintenance inspections I noticed the rear shocks started leaking. They have been replaced twice and are getting ready to be replaced again. We don't go off road with this vehicle so the only thing I could think is that the factory motorcraft parts are defective or not properly matched to this vehicle. If you see an oily residue on your shocks they need to be replaced due to leakage.