With 2WD I'd drop the loader first and see what happens.  If that doesn't fix it all, leave the fluid in and add cast rear wheel weights.  This is assuming that your Firestones are ag tires and not turf tires.  If turf then I'd get a set of ag tires if dropping the loader doesn't work.  Money's better spent on ag tires for ag work than cast weights.  Not sure how much you'd gain if you had industrial tires but you'd still be better off with ag tread.