Matt I bought mine at S&W in Lithonia, I know it's a little drive but mine was complete. Here is a few from a quick search on car-part.com. You might as well get one that has 4:10's from the get go since if remember right that is what gear you are still running.
1999
Rear Axle
Ford Explorer 11-98,4.0FI,AOD,SPOR T,4.10,8.8 93,000 A 6B0186 $100 S and W Automotive Parts, Inc. USA-GA(Lithonia) E-mail 1-800-706-8826 26
1999
Rear Axle
Ford Explorer 4-99,4.0SOH,AOD,4.10 9F0086 $100 S and W Automotive Parts, Inc. USA-GA(Lithonia) E-mail 1-800-706-8826 26
2001
Rear Axle
Ford Explorer 12 00,4.0L,AT,XLT 129,837 B HC0081 $100 Nemore Rex, Inc. USA-GA(Conyers) E-mail 1-770-786-9775 / 1-800-745-0272