im changing out a rear bearing and oil seal on the rear axle. it has c-clips

Oh, you are in big trouble. You will be forced to drill or grind the bolt out. Which will most likely damage the differential carrier. You will have to replace it because you've compromised the structural strength of the casing.

There are 2 tings to try first. Use a long pick and try to unscrew the piece that is still in the differential. Most of the time there is only 1/2 to 1 thread left and it will come out very easily. The second option is to drill it out. There is a special tool kit designed to drill it out. Snap-on, Matco, Etc all make it.