What rear end do I have?

[kitchene]

I have a 2000 f550 2wd 17,500gvwr and am trying to figure out what rear end I have. All of the stickers and tag is gone. all I have is the numbers on the front section of the diff. it's a front loader, and I think it's a dana 135. can anyone help me with this? It has 4.88 gears and I want to go to a 4.11.

Thanks in advance and if the VIN would help here it is..1FDAW56F7YEA57639