Boy, rear diffs are very difficult to fix.
Front ones are real easy on these cars, however.
Too bad it's not the front one.
You sure it's the gasket and not the pinion seal?
Been a lotta years since I did one, but it's a drop out third member isn't it?
You have to pull the wheels, pull the axle shafts, drop the driveshaft, unbolt the thrid member, drop it out the front.
Usually it's the pinion seal, but if you think it's the gasket, that's what you have to do.