Mike, I was having the same type of issues with my port side V-drive a few years ago. It is quite an undertaking to get to the seal. I would have atf show up in the bilge with no trace of where it was coming from. It only occurred when I would run up at cruising speed. If memory serves me correctly the seal cost around $20. The issue is you have to remove the exhaust. disconnect the drive shaft and pray that you are able to slide it aft. If you can't slide the shaft aft you have to unbolt the trans from the engine then slide the entire engine aft to be able to remove the engine. I think all told it cost me around $800 to have the $20 seal replaced. I didn't have the patience to tackle it myself so best of luck. (as a word of wisdom, It takes two people that can work together to put the exhaust back on the risers.) Good luck Mike, best of luck!