YFZ450r rear shock..

It is but you will have to cut off one of the mounts on the right side for the rear plastics.
that's on the trx450r shock. I believe the yfz rezzie hangs straight down, but they are a couple inches longer and require some modding to keep your ride height down
 
We see 400ex's on ebay for under $50 all the time, just wait for one, much simpler than the yfz
 
It's kinda hard to determine whiteout having it in person to see. Looking at the shock mount, the res is off to the side and back. Hard to tell by just a pic.
 
not sure how much cheaper you think you're gonna find a yfz shock ???
and you're sh*t's gonna look like a jacked up trailer park camaro with a yfz