Involved in the biosynthesis of the linkage region of glycosaminoglycan chains as part of proteoglycan biosynthesis (chondroitin, dermatan and heparan sulfates); catalyzes the reaction of UDPgalactose & 4-beta-D-(galactosyl-O-beta-D-xylosylprotein to yield UDP & 3-beta-D-galactosyl-O-beta-D-xylosylprotein