Can't tell you how many times I've seen the "less back pressure causes power loss" myth get thrown around on different forums. The simple truth is, too much back pressure is a bad thing.
And so is too little, which is the point made above and is definitively not a myth unless you have some evidence that the Liberty exhaust is too restrictive. Plus, "bad thing" is an oversimplification - the exhaust is tuned to match the valve timing and optimized for a particular RPM. Pull the exhaust of and you can sure make a heck of a lot more HP out of an engine... with the proper retuning. Likewise, a more or less restrictive exhaust can change how the engine produces torque and HP at different RPMs to meet a particular driver's needs, but the range is limited by the rest of the engine's tune, and I'd want to see testing data and the trade offs before making a change.
Regardless, I've done a full exhaust replacement on the Liberty. Not by choice, a rock found its way between the skids and made a mess of one of the cats and the muffler. The back half is a piece of cake, two nuts at the flange in front of the muffler and a couple of hangers and it'll come out. I may have had to raise the Jeep to twist the exhaust around the axle, but not by too much. The section from the downpipe to the muffler flange required lifting the Jeep and removing the crossmember, which means you'll also need to support the transmission. It's not too difficult, I was able to do it in my driveway, but you may want help if you haven't done something like that before, and you want to make sure you get the torque right on the crossmember when it's reinstalled.
FWIW, it's readily apparent when one of the downpipe flanges is loose, which has happened a couple times since I replaced the cats. I usually average in the mid-17 MPG range, and it'll drop down to 15. Of course, with a smashed cat and muffler I was getting a whopping 13MPG!