Shocks for Raptor 660

I'm looking to replace the rear shock on my 2004 Raptor 660. The stock one seems to have something wrong with the compression adjustment. I adjust it all the way to both extremes and I can't tell any difference. I was looking at purchasing a stock rear shock from a YFZ-450 but I have a question. On eBay the front shocks from a YFZ go for more than $200 but, the rear shock sells for less than $100. This seems odd to me and I wanted to know if anyone had any input. I want to replace mine because I'm bottoming the quad out on medium jumps. Please let me know why the rear (aftermarket rears are as or more expensive than a pair of fronts) are selling for so little. Also give me some input about how good an idea it is to put the YFZ shock on a Raptor. Thanks