Rear Diff Flange Nut

I'm trying to replace the oil seal and I cannot remove the nut, any tips?

First of all you need a torque wrench as you need to measure the current nut tension. There's another method requires marking the nut in relation to the pinion shaft. This mark needs to be accurate as the nut needs to be refitted in the exact place from whence it came. Once you have a mark or torque wrench ready to measure the tension, you need to make up a locking tool to stop the flange from turning. For this you'll need 2 flat steel bars with the dimensions of 1/4" thick X 1" wide and about 18" long. You need to drill a 10mm hole in each end, central in the width and about half inch from the ends. Next bolt the 2 bars together at one end with a 10mm bolt. Form the bars into a shallow v and bolt the remaining 2 holes into 2 opposite flange holes. This can then anchor the flange if you allow it to turn against something solid so the nut can be removed.

or you can just rock the pinion and gauge as you tighten nut once any free play is removed keep tightening carefully till you feel some drag appear

I had movement to start with so I could not gauge anything torque wise. After struggling I decided to make a locking tool which bolted to the flange and then a locking bolt into the lower bolt hole of the front mount, even with this it would not budge and it bent the locking bolt.

Make up the locking tool I described above. That's about the only way to hold the flange tight enough to shift the nut.