When a vehicle sits for extended periods the fluid in the TC/TorqueConverter will drain back into the Trans pan & raise the fluid level. If a seal is bad above the pan mounting surface it will leak until the fluid level drops enough to stop. If the Vent tube on top of the Trans is clogged it will also cause leaks from pressure buildup from temp changes.
The rear seal can be replaced by just removing the Driveshaft, prying out the old seal, tapping in a new seal. Use a large socket etc.. to prevent damage & keep the seal straight while tapping it in. Lube the seal lip before installing it.