Not sure if this will help on a 300K mile transmission, but mbz.org has a nice article on installing a K1 spring kit in the valve body to firm up the 2-3 shift:
http://transmission.articles.mbz.org/k1/

I've got the kit in hand, waiting for my next trans fluid change. As I recall, my local stealer wanted something like $15 for it.

Russ M