We've had our 2011 GL450 for 2 years and have about 136k miles. I need to replace the main battery. Scanner says my aux battery is fine, but I figures its the same labor and only about $70 so I would replace it at the same time.
Thoughts?

The only function of the auxiliary battery is to enable putting the transmission into Park if the main battery fails. It typically lasts a very long time because it does not do anything besides wait to be called on. However, even if unused such batteries do eventually fail - and yours is near its end of life.

It depends on your appetite for doing the work, or your appetite for paying someone else to do it.

There are generic versions of the battery that cost about $20. Some have unequal size lugs, and so the factory connector needs to be either replaced or squashed flat so it grips the smaller connector.

If you put a nylon strap ("webbing") under the main battery, it is much easier to lift out.

the original Aux battery in my 2007 GL450 was replaced in 2022 when replacing the main battery.
The original is a Mercedes Benz Rechargeable Lead-Acid Battery, N000000004039. 12V, 1.2Ah
I bought the replacement battery at Amazon: Casil 12v 1.2ah rechargeable sealed Lead Acid replacement battery for Mercedes Auxiliary Battery ($17)
It's been working fine.
And it's not hard to replace both batteries DIY. there are many youtube videos on it.