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Rear Driveshaft Slip-Yoke Eliminator (SYE)


Update: September 5, 2005

I have had the RE rear driveshaft off my Jeep for a month now. Amazingly, almost all of the vibrations that I have been enduring since the regear to 4.56 disappeared immediately so it looks like the RE shaft has caused me yet another headache. All I can figure is that the regear caused me to have to drive at a higher RPM for the same speeds, and that difference made the imbalance more noticeable. I'm driving around in Front-Wheel Drive until I can install a new SYE and rear driveshaft; the worst part of this is that because of budget and time problems, I have never taken the RE shaft four-wheeling so it hasn't been bent. It's just out-of-balance from the manufacturer. Coupled with the other driveshaft issues I've had from the RE kit, I really can't recommend the Hack-n-Tap and RE driveshaft in the future.

Background
The XJ rear driveshaft is a little different than many folks are used to. On the stock driveshaft, the main body between the u-joints is of fixed length. Since suspension movement changes the distance between the pinion yoke and the transfer-case output shaft, the driveshaft needs to absorb that change somehow.

Example of an XJ slipjoint/slipyoke

Slipjoint in the middle of a non-Cherokee driveshaft

On many vehicles, the driveshaft can collapse and extend through a built-in slipyoke. The output at the t-case is fixed in place and involves one or more u-joints to allow the shaft to rotate.

Since XJ driveshafts don't have that built-in slipjoint all adjustment is made through the sliding slipyoke at the top of the shaft, around the transfer case output.

the original fixed-length driveshaft.  Note the slip-yoke at the upper end
Late model 96+ Cherokee driveshaft. note the rubber boot around the slipyoke.

There are two main versions of the NP231 transfer case as used in Jeeps.

  • 1987-95
  • 1996-2001 (including TJ 97-present)
The earlier 87-95 versions of the NP231 had an internal slipyoke and female tailshaft that had no seal. If you had to remove the rear driveshaft the t-case would empty itself, so that wasn't practical or driveable. SYE designs for that era incorporate a seal to prevent leakage. The NP231 I have was introduced in '96 and although it introduced a seal to prevent leaking ATF on the trail if the driveshaft was removed, it still had a problem. Late-model t-cases in the XJ and the TJ are more susceptible to vibrations when lifted because they use a long, unsupported tailshaft. It's so pronounced that the Up-Country XJ models with a factory 1" lift had vibes requiring a transfer-case drop. I have seen dozens of people complain about vibrations in their late-model Jeeps after lifting, and ultimately the only fix is to install some form of Slip-Yoke Eliminator and CV-jointed driveshaft.

Options
Especially with the later-model NP231, simply dropping the transfer case isn't enough to fix the vibrations. For those folks, some form of SYE is needed. I originally used the Rubicon Express 'Hack-n-Tap' SYE, which is an easily installed, fairly robust piece of gear run by many, and which has lasted for years on many Jeeps I know of.

SYE and driveshaft installed
RE Hack-n-Tap with RE driveshaft on a 1996 NP231 t-case

The benefit of the Hack-n-Tap is that the installer doesn't have to open the transfer case; and in most cases the entire installation can be done in the driveway without removing the t-case at all! The downside is that some acclaimed driveshaft makers including Tom Woods will not build a driveshaft for a Hack-n-Tap equipped vehicle because of the possibilities of having imbalanced flanges or off-center cuts on the driveshaft. While there is an option to build your own driveshaft orfind the parts to make it, I'm not going to touch on that any farther.

Most SYEs for the NP231 involve opening the t-case and replacing the mainshaft with another. Either this is a completely new mainshaft that is harder than the stock one, or it is a stock one that has been cut and machined - you can usually identify this type because they require your old (usable) shaft back again. If you have any question about it though, ask the dealer or drop me a line and I'll see what I know about that kit.

The SYE I selected for Round II is of the latter type. I need to remove the stock mainshaft and replace it with a stronger unit. For this, I selected the Performance Offroad Center (PORC) NP231 SYE, and will document that installation here.

Other options:

Old Man's Budget SYE

e-mail Jim
created: September 9, 2003
Updated September 5, 2005