Thanks guys. I tried pulling on the plug with a set of vice grips whilst spinning it. No luck...
Tried heating with a blowtorch and repeating. No luck...
Then I went to bed.
Got back underneath the ute this evening, removed the diff and had a look inside.
Tapped the plug with a hammer, and then started trying to untighten. She bit on the thread slightly, but not enough to come off.
Ended up pressing down on it whilst I turned the plug and she bit and came out. Winning!
The good news is that the thread in the axle casing looks pretty good, and only the plug is stripped.
Gonna go down to Toyota spares and get a new diff plug and see if she fits the thread. Fingers crossed she will! Otherwise I'll buy a thread cutter and tidy her up.
Got some Permatex ultra grey in the shed - will that be ok for the gasket? Or is there an actual gasket to use?
Thanks for the tips guys!

