I've been driving the rental for about a day now, and I've realized two things:
one, this rental has 2000 miles on it, and it doesn't really shutter much, but ocassionally it will, which I'm starting to think is somewhat normal for these transmissions (like I said in another post--these are a new type of trans that many are not use to).
I did notice in my personal car that the shuttering got better (lessened) as it began to accumulate miles. I may still see the dealer to re-program the trans if they don't end up replacing it--or at the very least "reset" the transmission so it can relearn itself. If it gets worse, or persists then I'll have it look at again, but I'll give the car up to 2k before I go to this step.
My hope is that if my problem ends up being a trans problem, that they would just swap it out for a new one--since after all, this seems to be an odd problem according to all of the techs I've taken the car to. That would be my preferred route that I'd like to see them take.