I think you'll need to find out what drive train is in it. If it's all stock, shouldn't be a problem...
Find out if it's the front or rear, then start checking online for best pricing...
First google search for "Driveshafts for 81 CJ" brought up Quadratec's site, although the product it brought up looked like "Tom's custom Driveshafts" which was $210.00 for the front one... Most likely a lot cheaper versions somewhere... Tom's sells online too, so might get cheaper that way? You'll have to do some research for best pricing once you know what you need, and what you have...
Otherwise, if it's not stock drivetrain, you'll have to start figuring what you need in another way... Myself, I'd take the old broken one locally to a shop and have replacement ordered / made depending on the drive-train present.
Welcome to the site man...
~ JR
