you have a cone type sure grip -- thrust button shown in the pic is correct for the cone type sure grip and looks to be ok -- looks like the pinion bearings gave up or the yoke nut backed off and the pinion was chewing on your carrier -- the pic you posted of the backing plate shows the turned up edge of the parking brake strut which i have only seen on stock A-body backing plates . B,C ,E body backing plates have a flat parking brake strut.