Anyway, I'd hate to box up the Toshiba, return it, get the Insignia,
and find out that it has the same problem. Maybe I should just buy
the Insignia, and then return the one that I like the least? Or should
I just learn to live with the issue in the Toshiba? (only happens with
that TV, no others in my house)

You need to find out what's going on, and make a rational decision, rather
than schlepping boxes.

I would start with the other TVs, and take notes as you watch. Exactly what do
they to do to the picture (or not do)? Then compare them with the Toshiba.

Remember, the guy at Best Buy wants to put out the least amount of effort to
get you to buy something. It's unlikely he'll be willing to help you make the
right choice. It's in your hands.