I've never seen that. I thought you were talking about driveshaft spacers like some people use on Suzukis.
That's the problem, I haven't seen them either; but it seems like such an easy idea (would make the front driveshaft lengthing easier when you get a crawler too).
Then you don't have a custom length driveshaft, and if you ever need a new one, just goto your local junkyard for a replacement.........