I don't know about Ms Diller as Elphaba but I adore my two Phyllis Diller comedy CDs. They sound a little old fashioned at first but after a few minutes the time melts away and she has me in fits of laughter.
Some day it might be fun to hear her cackle as Elphaba, or maybe she could play the aged Glinda in the further tales of OZ.
" ...the happiness in the tune convinces me that I'm not afraid."