How to isolate a refrigerator?

Turn off the fridge when you listen to the rig, or just dump the fridge and go with the rig. LOL

Here's what I do.
I have my fridge plugged into a light timer (Home Depot)with a on/off switch. When I do critical listening I hit the switch. So for about an hour no fridge. When turned on it only losses 2-3 degrees. At 2 hrs it losses 5-7 degrees. Which is not alot.
Just remember to turn it back on.

Have you tried Silent Feet?

http://www.vibrationsolution.com/index.php

Depending on the age of the fridge, you may have some compressor mounts that need replaced and that could reduced the transmission of sound. Also try foam insulation in the ceiling below the fridge if accessable.
If not accessable try drilling a hole in the floor in the middle of the fridge footprint area and blow in insulation either foam or dry type. That patch for the hold won't be visible unless the fridge is out.

in terms of upgrades, if you can't fix the problem with isolation (like feet or cork), buying a quieter fridge is pretty cheap - you might even find one for free on Craigslist. I have a 7 or 8 y.o. kenmore that's extremely quiet - the neighbors gave it away when they remodeled their kitchen - and it's much quieter than my previous one.