Help on a Cat 953

I have a 1989 Cat 953 loader. I was operating it the other day and I was going forward and stopped, when I pulled the leaver in reverse I heard a loud pop. Sounded like something broke, sounded like it was in the right final drive somewhere. The loader will not move in either direction. The bucket hydraulics still works. What is the best guess to what happened? Anyone have any suggestions how to proceed from here?

Help on a Cat 953