Just looking for others’ opinions on the rust I’m seeing on my rear axle shaft. Problematic or no?

Rust

No rust

 

Not a mechanical expert but it looks concerning.

 

Surface rust. Not concerning as long as you treat it.

I would use POR-15 to coat the rust.

If you have an early car, I'd be painting the steel underbody braces while you have the paint can open. In particular, check out the areas behind the wheels...road debris can wear off any coating, even if they are the newer, properly-finished ones.

++++++++++

Some makes have rubber dampeners on the middle of the drive shafts. On those, the axles can actually break if the rust penetrates. The dampeners keep rubbing...eventually they twist apart!

 

I must say your car is very clean underneath!
.
.
.