Difficulty in extracting one from a junk yard would depend on whether the junkyard owner would be willing to part with just the glass or require you to buy the whole liftgate. If you could get just the glass I would think the most difficult part would be getting the junkyard window off the donor liftgate without breaking it. It looks/sounds like it would require a certain amount of time and patience and probably a buddy to help you. As for installing, it looks like the hard part would be getting the glass on there and sealing it properly without dropping it. Again, a buddy would be helpful here.
Have you checked with your insurance company to see if they will cover the glass? If you absolutely must make this fix yourself, you might want to consider replacing the whole liftgate if it doesn't cost that much more.