should I change gears or tranny???

I would go cheaper and just gear it if the trans., etc. seems to be solid. Might be kinda difficult to find the ratio you want if doing the junkyard crawl since most D60's seem to come with the 4.?? gears for towing. New of course you have tons of options.
The trans. would be the best overall. I would just pick up an early one, throw a rebuild at it and toss it in there. Just don't forget about all the other B.S. you're going to need with it. I think they came out about '90 in all the full size vehicles.