TH350 is bleeding!

[383drvr]

The 350 transmission in my 83 is leaking from the tailshaft area. But it only leaks intermitantly. Not necessarily when its on a hill, it leaks sometimes when the truck is perfectly level in my backyard. It will go weeks and months sometimes before "purging" almost a quart onto my drive

So, Do you think its a rear seal even though it leaks on and off? I've had this truck for a few months now, and the previous owner (mechanic), said he just replaced it...

If it is the rear seal, is it something I can fix myself in a few hours or over the weekend? Its my only driver.... What is the process in replacing this part?

Thanks!