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SYE is an acronym for Slip Yoke Eliminator from companies such as advanced adapters and tom woods drive shaft. It removes the slip yoke from the rear of the transfer case and replaces it with a fixed yoke, adds a lot stonger output shaft, adds about 5 inches to the drive shaft length, adds a cv (constant velocity) style shaft, reducing the angle of the shaft and reducing vibration. Commonly found on jeeps with 4+ inches of suspension lift. Another added bonus of these SYE is that you can remove the T-Case drop and increase the ground clearence.
Do a search in like yahoo and you will get a hundred million sites about instillation and performance.
Johnny[addsig]