Nope- - - -it's just being locked by the remains of the cotter pin. Take a 1/8" drill bit to the cotter pin and get the rest of it out of the hole, or use a pin punch and a big hammer to drive it out. I'd prefer to drill it out. Then grab the pinion flange in a BIG pipe wrench and brace it on the floor while you use a 3/4" drive socket, a breaker bar, and a couple of feet of pipe to loosen the nut. Last resort- - - -heat the nut with a torch and apply lots of penetrating oil. It's not uncommon for those pinion nuts to have 200+ ft/lb of torque or more.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!