On the rear caliper, the top slider is free (moves easily by hand) but the bottom slider is seized. I've removed the rubber boot, lubricated with generous amounts of WD-40 and PB blaster, and tapped on it w/ a screwdriver & hammer...no success. Any tricks?

I stole this pic from another website because it looks similar. One of the bolts threads in where the wrench is...and that piece is supposed to slide in & out so the caliper can float.

 

keep at it with the WD and the hammer. Persistance is the key.
If you are convinced that it won;t come without breaking, heat is the next option. Heat the bracket up with a torch and try to work the slider free

 

With the rubber boot gone, it should come all the way out? Then I can buy a new one, grease it, and finish the job?

 

You must live in the north with rust like that.

Did you ever get it out?

 

Air chisel is what we used to get mine off.

 

lots a rust huh ! gotta prety good idea where that truck grew up! But hey these brake systems really do suck pond water! if there is that much rust then take a good hard look at the piston they are notorious for getting so much rust build up that it actualy will force the pad against the rotor unable to release i didnt believe it either till i saw it ! when you replace the caliper you might want to not only take a hard look at the other side but work the piston surface back to shiny metal, then apply a quality rust block .another thing pull your soeed sensers clean them up then apply a good rust eater and spend some real time cleaning the inside housing . apply grease to senser housing and reinstall. rust in the senser housing can play havack with abs modules throwing codes and sometimes not and reports of loosing braking ablity are common. all around not gm's finest hour