I recently found out they start in second - so I stand corrected on my initial post.

However, I did attempt to drive it for the first time since this happened and it's still doing the same thing. It'll run in 2nd up to about 4500-5k rpm (40-45mph) and it only shifted into 3rd ONCE.

Did another check of the trans. fluid - before and after the drive. Before it was showing right above the 1st notch (car sat running for about 10min) then After, it was showing right above 2nd notch.

I was advised on another forum (benzworld.org) to check vacuum lines - but have NO idea of where to even start w/that.