trans help

[6.0joe]

I have a 2002 F-350 diesel, it has 50,000 original miles. I was stopped at a stop sign, and when I pulled out all seemed ok, 1st gear was fine, 2nd gear was fine, but when it shifted into drive a loud bang accured, and it seemed to lock up. 1st and 2nd gear seem fine, I also have reverse. But when it shifts into drive something is wrong.