well first i'd check u-joints that clunk could the u-joints are on the way out crawl up under there and give them a bit of grease but i'd get those changed out.

320k is pretty high and is about time for that tranny to get a rebuild I think lloyd's 93 had about 300k and his auto was getting pretty slow on the shift. When was the last time you had the fluid changed out?? i'd pull the dip stick and check the color if its brown its time to change that fluid out either take it to a lube shop that will pull the tranny line and drain the whole lot of fluid or what i've done is drain it drive it for a bit change it again usely takes about 4 times of draining and driving before i get all the old fluid out.

I'd stick with the auto its a good strong unit i think its just time for a rebuild on yours.