Brand new tires are likely to still have some of the mold release agent in/on the tread rubber. Don't expect them to perform as intended until you've scrubbed it off. This should occur within a couple hundred miles of normal driving, and that's arguably the better way to get this done. Get this done and see if the problem goes away.
There isn't enough information for me to suggest a different rear tire pressure.
This. I just put 4 new tires on my car and noticed the exact same sensation when driving it home. After a few days of driving to and from work (and some spirited cornering for good measure), the car drives just like it did before replacing the tires. Give it a few days and see if it's still a problem.