Please note: If you have to ask how to balance one of these or if they vibrate at speed, you are probably not going to be in the market to build (or want) a square drive shaft....
Here are some links showing how to build up a square drive shaft for a spare or wheeling only use:
http://www.rocketcityrockcrawlers.co...%20Project.pdf
http://www.pirate4x4.com/forum/showt...ghlight=square
http://www.greatlakes4x4.com/showthr...&mode=threaded
These are also pretty good:
http://home.4x4wire.com/erik/4runner/sas/#driveshaft
http://www.pirate4x4.com/forum/showthread.php?t=90517
http://www.ehow.com/how_4475628_buil...riveshaft.html
This one is really a good write up:
http://infohost.nmt.edu/~jscarbro/driveshaft.htm
Here is another good one:
http://board.marlincrawler.com/index.php?topic=14915.0
And, surprise, surprise, a bunch of really good info on the 'squaredriveshaft' site:
http://www.squaredriveshaft.com/