While getting an oil change at the local Valvoline outlet the tech mentioned that I was overdue to have the transfer case oil changed...we have a 2007 ML with 135K miles

1. Was he right?

2. Should I go to the dealer to have this work performed?

As always any steer from MB World experts is welcome...

Ok, so I'm going to assume by transfer case he means transmission fluid. If so then I would say "oh yeah". You are way overdue...especially if it's never been done. The manual states every 40k miles but assuming MB used synthetic ATF in the tranny's then you could easily go 100k miles. So you're at the very least 35k miles overdue.
If you go to the dealer it's gonna be pricey. If you can find a certified MB Indie Mechanic you can save yoursefl a couple hundred dollars.