What would cause the speed sensors to keep breaking? on 2004 Hyundai Accent

It's the speed sensor that go's into the tranny. It was broke when we took it out. Then we replaced it and it's broken again. The car is also making a clunking noise when the tranny has load on it. It also is shifting fine so what do you think would cause them to break and are we looking at having to get a tranny?

Asked by for the 2004 Hyundai Accent
Installed correctly? What part of it is breaking the shaft, the threaded part, what?
When we pulled the original one out half of the black plastic was broke inside the tranny. We put a new one in and after a few mins of driving the speed wasn't working again. So we're pretty sure it's broken again. Do you know what would cause them to break? Could something inside the tranny be off balance?