RE website FAQs:
Support and FAQs - Rubicon Express
"Which drive shaft do I need to order?
The drive shaft style you chose is dependent on the SYE kit you choose. For the RE1801 & RE1807 you need the RE1898 series shaft. The RE1811 SYE kit will require the use of the RE1860 series drive shaft. Rubicon model Jeeps require the RE1895 "direct fit" drive shaft.
How long of a driveshaft do I need to order?
The drive shaft length is determined from the length of your stock drive shaft. To measure your drive shaft you need to measure from center to center of the upper and lower u-joints. If you order a RE1801 or RE1807 you will need to add 5.5" to your drive shaft length for the RE1811 add 4.5" to you measurement. Example, If you measure your TJ wrangler stock drive shaft at 11" and your installing a RE1807 you would need to order a RE1898-165. If you measurement does not match exactly, round down to the nearest size for a short arm kit and round up for a long arm kit. For YJ, XJ, & ZJ always round up to the nearest size."