Jeep dana 35 rear end all seals blown?

my jeeps rear blew 8 months ago i swaped for a salvage rear wich has worked great for now but my girlfriends said that there is a klunk when she hits bumps to me sounded like a bushing couldnt find a bad one yet still have to get it up on a lift but when i looked under neath both wheels seals and pinion seals where leaking what could have caused that what should i do replace both bearings and seals since there presssed on axle or swap a new rear help what could have caused this