2003 Mercury Mountaineer clunking sound from the front

I have a 2003 Mercury Mountaineer that is making a loud clunking sound when I back out of places. I recently took it to get an estimate & was told that it was the axle, when I took it back in for the repair they called me back & said that the axle was not the problem, it was actually the bearings & sensor on the driver side (they replaced them) after I got my truck home & backed out of the driveway the clunking noise was still there. After a very heated phone call the agreed to pick the truck up & fix the issue with no extra cost & they even put me in a rental (2weeks) while they figured it out. They replaced the axle (still noise), transfer case (still noise), drive shaft yolk (still noise), differential (still noise) & it seems like he told me another part. He called me & said "I hate to say we failed but we failed we can't figure it out & we can't fix it. I previously had both axles, bearings, hubs & ball joints replaced. Please help!!