Must-do on any lifted XJ with NP231. Converts the slip-yoke output to a fixed yoke, allowing a CV driveshaft. Choose between cheap 'hack-n-tap' or full HD kit.
The NP231 slip-yoke rear output is the single biggest weakness on a lifted XJ. With more than 2-3 inches of lift, the slip yoke binds and pulls hard on the t-case rear seal — leaks, vibration, eventually output bearing failure. The fix is SYE: replace the slip-yoke arrangement with a fixed yoke and run a CV (double-cardan) driveshaft.
Two options:
1. Full HD SYE kit (Advance Adapters 50-7901, JB Conversions, Novak, Tom Wood's, Rubicon Express): replaces the rear output housing, mainshaft, and yoke with a heavy-duty 32-spline assembly. Best for built rigs, mandatory for stroker/V8 power levels. $300-500.
2. Hack-n-tap (Rubicon Express RE1655): cuts the existing OEM mainshaft, drills and taps the cut end for a yoke bolt. Cheap, fast, works fine for street and mild trail. Not as strong as HD kit but stronger than the stock slip yoke. ~$150-200.
For 96+ XJs the hack-n-tap is especially popular — saves ~$150 over HD kit and works for most. For 87-95 XJs the hack-n-tap is harder because of mainshaft geometry differences; HD kit is recommended.
Install: t-case can stay in vehicle for hack-n-tap; HD kit more manageable with t-case on bench (3-4 hours work). Both require new fluid, new rear seal, new rear driveshaft (CV, see drive-cv-driveshaft entry).
Advance Adapters is widely considered the best HD SYE. Tom Wood's bundles a complete kit with CV driveshaft to your length.
Flag: this is a MUST-DO upgrade for any lifted XJ. Driving lifted with slip-yoke chews seals and shortens t-case life dramatically.
| Part | Vendor | Est. price |
|---|---|---|
| Advance Adapters HD SYE | Advance Adapters | ~$380 |
| Rubicon Express hack-n-tap SYE | Rubicon Express | ~$180 |
| Tom Wood's SYE + driveshaft combo | Tom Wood's | ~$650 |
| Novak SYE | Novak | ~$420 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.