98 Chevy Express Van speedometer reading very high

I have a 98 Chevy express van with about 100k miles on it (305 auto trans) I "DID NOT" change tires gears ext. for about a month the trany would sporadically not down shift. Now the problem has become consistent along with the speedometer now reads about double the speed (50 mph when your doing about 30) this of course causes poor shifting and the ABS light to come on. There are no error code's (I can get to) nor is the check engine light on. So I changed the speed sensor which changed nothing. Any Ideas