I had my rear shock rebuilt when I bought new fronts. I got TCS's with the zero preload and the rear needs to match, the zero preload that is. The rebuild was about $450, new dual rate springs and valving. They do a stock rebuild fopr around $300 I think.
There was a marked improvement in back but not as great as the fronts, they were changed from stock to +2 arms and long travel triple rates with rezies. A bit of a change at the very least.
If your only bottoming out on occasion thats fine, you should. Then you know your using all your travel.
If I was choosing between the rear rebuild and saving for new fronts, I'd wait with the rear till it was necessary. The stock rear is a respectable shock, the fronts are not.
With what your looking to spend on the rear wait and find a set of YFZ's, not my personal experience but I've never heard or read of anyone being disappointed in the YFZ upgrade.
Search around for a set of longer a-arms also.