X-heads vs J-heads: what's the difference??

Well as humans we make mistakes. I know a guy who bought a new 2001 Ford Ranger with the 4 L SOHC engine. It was 4X4 nice little truck. He had it a while and the trans acted up check engine light etc. It was still under warranty ,he took it in, they told him it had a part missing inside the trans from the factory, I forget whaT IT WAS BUT IT WAS LIKE wHA? ITS AMAZING IT RAN FOR A WHILE BEFORE ACTING UP...sorry caps lock :)