Hey, Im an experienced guitarist with a GCB-95 wah wah pedal, and I'm woundering how zakk wylde does his screaming wah bends such as the one he uses in his solos in 'in this river' and 'farewell ballad'.

What type of technique is used on the wah wah pedal to make the sound?

It's rather technique than the wah pedal.
zakk is probably using pinch harmonics to make those screaming wah bends, combine this with the wah in forward(treble) position and you could get that aggressive screaming sound like zakk.