transmission leaking

I have a 89 f150 5.8 litre 190000 miles c6 trans. It is leaking some fluid out from it looks like around the tail shaft.It leaks around a quart every 600miles,which isnt all that hard to addonce a week as it is my work truck this should be fine as long as i keep it within operateing range,right?? can i pull my drive shaft an replace that seal or is it a drop the tranny job? It shifts smooth as silk an never slips , though she is getting some miles , can any one give me a ball park on how much $ a rebuild on this trans would cost me for down the road if i need to ? thanks a lot