The only CTDs that had a 61 axle were the “W” (4wd) and they were front axles. All pre overdrive CTDs had rear 71s otherwise known as D70U. The only difference from a standard 70 was an internal “U”ndercut for the large pinion gear tooth clearance. Not a good rear for performance by any means. The reason I know this is I have seen more than my fair share, and had to source parts for them.😉