Must be frozen in there with old crud or they're corroded. I'm not familiar with PB blaster, but I know good ol' WD40 dissolves crud really well. Maybe jiggle the shifter a bunch to work it free, and then give it a good yank? There's nothing fragile in there, just the main mechanism and the two side plates on either side that interface with the slots on the inside of the body. Are the shifters installed in the handlebars now? If not, possibly you could tap it gently with a punch or something from the backside.