I'm in the process of swapping my rear end. Here's some tips so you don't make the same mistakes:
I checked the tag on the rear end and got the number.
I then used this chart: Ford Rear Axle Assembly Identification - Page 13 - FORDification.com
I have a S409-H tag which is the Ford F250SD, F350SD Pickup ‘00-01 3.73 10.50 LSD.
I got a junkyard special with the S461-H tag which is the Ford F350SD Pickup ‘00-01 3.73 10.50 LSD.
They looks like they'll both work right? Not quite.
Tip 1: Measure back plate to back plate to ensure they are both the same width. These just happened to both be 60".
Tip 2: Measure back plate to the leaf spring plate on both sides. My original ones were 40" inside to inside. The new rear end was 38" inside to inside. We are now grinding off the welds and re-welding them to the tubes further out.
I'll update as we get it swapped out.