Alan,
Ran into the same problem. I needed to rotate tires anyway so I decided to remove the rear drums in the process... to see what I was doing wrong. I proceeded to adjust the rears as if the drums were still in place. . Only now I could view what I was doing wrong. Not a problem since.
------------------
98. 5' 24v 2500 Auto/3. 54 4x4 SB QC 285-75-16's on Alcoa's. Everything but leather. PIAA 1200's, AMSOIL dual filter relocation system, Smittybuilt Stainless Steel Nerfs, Rhino Liner(Junk), K&N air filter(For Sale)15 year AMSOIL dealer. Time permitting, Soon to include Ultra-lite Pyro,0-50lbs boost, Trans temp in the pillar. Rancho 9000's with in cab adjustment.