First off, wipe down every seal, joint, and fitting that is wet or dirty. Then drive around block look for new fresh fluid. to find location of leak. Rememeber gravity and air flow will carry the fluid down and back.
The rear seal on the transmission might need replaced. These seals go bad. They consist of a metal ring (that fits into the end of the
trans.) with a rubber seal going around the inside to seal around the drive shaft. They are only a few $ at autozone.
To change one: remove the drive shaft and pry out the old seal (just use a big flathead screw driver to pop it out, don,t worry about destroying the old seal, it will happen apon removal). Carefully and evenly tap in the new seal (with a big mallet or something heavy and non-marring) in place till seated fully onto the
trans. housing.
If not that then try replacing the gasket on the pan. just buy a transmission filter kit they come with new gaskets and replace the filter while your at it (kits are around $20 at autozone).