Is it engine oil or transmision fluid? if it's engine oil (most likely) than it usually is the rear main seal (seals the crankshaft/case), The seal is cheap, but getting to the seal requires engine removal (assuming you can do that yourself, it's not to much fuss replacing that rear main)
BUT... i would recomend cleaning that area real good (almost spotless) so you can pin point the problem (break clean looks great for this!). It could even be the case halfs, oil plugs...ect
Just think of all the deatailing you could do with the engine out though
! HOW FUN!!!
Jonathan