2004 35th Anniversary Wagon Front Differential Seal Failure

[LouSubu]

Hello - New to the group but, long time Subaru fan. I recently inherited a 2004 Legacy Wagon and was in the process of bringing it up to some sort of baseline maintenance, It was a family members and much maintenance was ignored or neglected. After doing a few routine maintenance things like oil change, coolant, etc. I decided to drive it to work to keep the miles down on my Toyota Tacoma and save some gas. The car is in relatively good shape (though it does leak oil and I suspect the main seal is not long for this world) and I decided to use it as a couple time a week driver to work and on the first trip... loss of tranny engagement, high rev... couple of blocks and won't move. Runs fine, I suspect I blew the differential seal from the looks of things and the tranny fluid was bone dry. My question is what did I destroy if anything? Still runs fine, just a big puddle of fluid under it. Had it towed today and plan to do the work myself this spring. What should I know? What needs to be replaced? Can I put a new seal on and go? I suspect there had to be a reason it failed. Help please. Thanks in advance.