By slip joint I am assuming you are talking about the box that connects the steering shaft to the steering box. If not disregard the following. If I am right I would upgrade to a universal joint. I believe Borgeson has one. Here is a universal and shaft kit but I think the universal is available by itself.
http://www.jegs.com/webapp/wcs/store...294966160&Ne=1 147708&redirect=false&searchType=MkMdlYrSearch

 

I have seen the rebuild kit for around $10, but no one has it in stock.

 

if you do rebuild the slip joint, get yourself a C/V boot off of some sort of a car and install with a coulpe clamps to the shaft and the joint, will keep the crud out and makes them last a little longer