[Jah Rider]

See here for a method to hold the pinnion whilst undoing the nut. It`s just a simple steel strap although you could use a short length of strong chain,too.
You`ll have to do a partial reassembly and use the drive spline from your rear wheel whilst using your lock-up tool.

https://ofapars-.webs.com/solofinal.htm

If you want to go the OEM route the pinnion holder is still available.
It`s $754.

https://www.cmsnl.com/products/07924-me40002-holder-pinion_07924me40000/

 

[Jah Rider]

Those nuts are fricking tight, i can`t see anything printed withstanding the torque involved in undoing them.