I had something very similar happen to me about 3 weeks ago. I was cruising down the 401, doing about 120 KM (70-75mph) in my Explorer when I also heard a large bang, felt the front end drop and the vehicle lurch towards the left. Fortunately, I had both hands on the wheel so I was able (with a lot of corrections) to keep from hitting the shoulder or going completely sideways. I first thought that I had blown a tire, but when I finally got the truck stopped after a very scary ride, I got out and found the front left tire sitting with extreme negative camber. I had also broken a ball joint, but mine was an upper. It had sheared off right at the ball, similar to yours.
I had also replaced my ball joints about 2 years ago and maintenance has always been done regularly. I suspect the problem was the (probably offshore) replacement parts, since this vehicle has not been abused and was traveling on a smooth highway at the time of the breakage. The ball joint has been replaced with a genuine Ford part this time.