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.........