I swapped in a used 4eat auto transmission in my 1996 Subaru Legacy and I drove it 30 mins no problem then I got out of work then I checked the transmission fluid which was fine and the differential fluid which was also fine so I started to drive home and I had a white smoke screen behind my car so I pulled over and transmission fluid was everywhere and it looked to be coming out the breather valve so I check my transmission fluid which was obviously low and then I check my differential fluid in it was full to the top with ATF. 
What could be causing transmission fluid to get into the differential? 

Also what is the name of the part that would cause ATF to get in there?

Edited November 11, 20214 yr by 3rdgearfun

Worn seal. Happens a lot on those 90’s automatics from my viewpoint. 
 

It will require the transmission be removed. 

Seal/oring between trans and diff.  Sounds like you had an exceptionally rash failure. 

Does anybody know what the seals name or part number is?