I juse spent 45 minutes looking and came up with the same results as you. It seems no one makes a rear diff brace for the Grizzly 700 anymore. You can still find Rhino braces, and a few 660 Grizzly braces, but no 700's.
Rage Cycles only makes Polaris SxS stuff now.
I tried Turner Cycle's website, but it sucks. It has nothing on it of any value. I do know they used to sell a diff brace. They probably still do, but you need to call them to find out. Get the number from their website.
The other 2 options is to make one yourself....or get one made at a metal shop. Show them pictures of what your wanting (with the bike with you if possible), and they should be abe to quote you a price.