Small width tires all around

River4,
Welcome to the community. Take a look around and don't be afraid to post if you have a question or need help.

Here is a few topics to look at.

See you are a jeep owner. I'm guessing you have the ability to do some welding.. If so you can modify the EMP rear bumper to fit with the P4 like I have done. Here's a few links:
Modifying the bracket to clear the rear ROPS - pioneer-700-accessories-and-modifications-f48/emp-rear-bumper-extreme-metal-products-t1243.html
Installing the EMP Rear Bumper - pioneer-700-accessories-and-modifications-f48/emp-pioneer-extreme-rear-bumper-installation-t1288.html

As for the mud... Well, this is common with aftermarket wheels. We ALL are waiting on some aftermarket mud flaps.
Until then... Here are a few post from a few talanted members.
Front - pioneer-700-accessories-and-modifications-f48/fender-flares-t1216.html
Rear - pioneer-700-accessories-and-modifications-f48/rear-fender-flares-t1012.html

As for going smaller on the rear. Not sure that would give an improvement you fell good about. You actually need to change wheels to a lesser offset.

Thanks.