Your issue definitely sounds like grease in the shoes. I'd remove the drums,clean them and maybe have them lightly turned or replace them w/NAPA's premium ones and their shoes. If you get new drums,the machine shop guy recommended a light turning even when new. Sure enough they were slightly out of round brand new. My problem was just getting that truck to stop. I did the EGR rear disc setup but if I were to do it again,EGR sells a performance complete brake system which should greatly enhance your stopping power. The duallys as I remember don't have a proportioning valve to begin with. The braking problem of those trucks ended with the 2001.5's discs. I hope I helped you. My# is 1-619-507-1020 if I missed anything. You do have the hydroboost master cylinder in the '98 which is great. Dave