Question: I have just bought my 1st Jeep Wrangler – 1991 and it has a number of 
modifications already. It has a Rubicon short shaft conversion and I am wondering what exactly this does and how it benefits the Jeep? I have a knocking when pulling away in lower revs in 1st and 2nd gears and maybe you can give me suggestions as to what to look for? I look forward to your help. I am based in France (Europe). Cheers, Mike
Answer: A short shaft conversion bascially replaces the original slip yoke on the back of the transfer case. A slip yoke simply provides the means for the drive shaft to extend and retract during suspension movement. The downside is the slip yoke is limited in length and with a suspension lift the drive shaft can actually fall out of the back of the transfer case.
See this article: What is a Slip Yoke Eliminator and Why Do You Need It? for a better definintion. Another good article: Transfer Case Upgrades, Modifications, and Options.
Regarding your knocking, I’d suspect the transmission over the transfer case. You may have a broken gear in there. Probably wise to check out.