I'm throwing this out there in case someone else is interested in the information.
The company that is supplying some Costco stores with kosher beef, Colorado Kosher, uses cattle that are grass-fed for the first part of their lives, then finished on a feed lot (e.g. grain) for the last 4 months or so. So they aren't 100% grass fed, but they are a combination. I got this information from the company, when I called to ask.
So, if you wanted to try the taste of grass-fed beef, and keep kosher, find out if you local Costco store stocks this brand.