"Chris" wrote
Is there any reason to take off the entire stern
drive to replace the impeller or should I just take of the lower unit?

You can replace your impeller with the lower unit removed and the upper
still on the boat. I just greased my u-joints over the winter while
replacing some other stuff in that area. I'm pretty sure the last time they
were greased was 27 years ago when my drive was manufactured. (I got it
second hand two years ago and the previous owner doesn't appear to have been
a big fan of preventive maintenance.) I don't have my manual handy, but I
suspect the recommended interval is a little shorter than that.