Nice Tailshaft

You’ve just slapped down a chunk of change on a new lift kit for your Jeep and spent several hours covered in grease and road grime on your day off installing it. Your Jeep went from a plain Jane to a stout-looking rig in just a few hours of wrenching. It looks damn good, and you know it will ’wheel better, but now when you drive your butt gets all numb from that nasty vibration coming from the slip-yoke on your rear driveshaft. The lift company tells you that this is fairly common on Wranglers and XJs with lifts. They offer some solutions like a transfer case lowering kit or shims to adjust the angle of your rear pinion. Lowering the transfer case sounds like it defeats the purpose of the lift, and you want to avoid removing the rear U-bolts to add shims since your back still hurts from the install. Furthermore, how much change can a little shim make, and how exactly is your drivetrain going to be realigned after you lower your transfer case a ½-inch? Don’t worry, there is a good solution that doesn’t involve shims or spacer pucks, and it will permanently cure more than just the vibration. It’s called a slip-yoke eliminator kit, and there are several available to suit your needs and your wallet.

The problem with the slip-yoke design is that it requires a shorter driveshaft to make room for the area on the yoke where the compression and extension take place. This means a mini driveshaft on the short-wheelbase Wranglers, and shorter-than-optimum driveshafts on Cherokees. In stock trim, the slip-yoke system works fairly well, although we know stock Wranglers come from the factory with a little vibration. Another shortcoming of the slip-yoke design is the force of the compressing suspension, which causes accelerated wear on the splines of the slip-yoke housing. As the rear suspension compresses, the slip-yoke moves into the housing to compensate for the differential coming closer to the transfer case. When this happens, some of the force of the compression travels up the driveshaft and pushes laterally on the tailshaft. The same forces push on a fixed-yoke–style output, but it is bolted in place and does not have to move in and out like a slip-yoke. This wear can result in failure of the seal and dramatic death of your transfer case if all of the lubricating fluids escape.

The third problem associated with the slip-yoke design of the NV231 has to do with a weak point in the mainshaft of the factory unit. In this area, the mainshaft tapers down where the spiral-cut gears mesh with the speedometer gear on the sending unit. This thinner tapered area is where the mainshaft may fail under extreme torque load or driveline bind.

Low-Buck Kits
You sure can’t beat the price of the tap-’n’-drill–style slip-yoke eliminator kits. These kits generally come with a new yoke that can be bolted to your factory mainshaft after it has been cut to a specified length, then drilled and finally tapped so a retaining bolt can hold the new yoke in place. If you want a lift for your Jeep but don’t want to ’wheel it hard, this could be the ticket for you. The kit won’t eliminate the factory weak point of the stock mainshaft, but it will at least get rid of that silly slip-yoke thingy, and give you more room for the driveshaft. As for installation, it sounds simple. However, cutting your mainshaft means no going back, and drilling a perfectly centered hole in your now-cropped mainshaft could prove difficult for the driveway mechanic.

Midrange Kits
Most companies selling heavy-duty slip-yoke elimination kits also sell a mid-range, or non-HD kit. These kits are similar to the tap-’n’-drill kits, except the companies that sell them do all of the machining to a stock mainshaft in exchange for your usable core and a credit card number. These kits are generally a bit more expensive than the low-buck ones, but you don’t have to worry about cutting your mainshaft to a precise length or drilling a hole to tap. Some companies also offer a mid-priced tailshaft conversion where they add material to the stock shaft and then respline the unit to add a little strength. Most of these kits require disassembly of your transfer case just like the HD kits for installation. So if you don’t want to play driveway machinist, this may be the way to go.

High-Dollar Kits
Most of the expensive kits have been reengineered to ensure that the new components will strengthen your transfer case while eliminating the slip-yoke, as well as providing more room for a new, longer driveshaft. The higher price is usually reflective of the quality and definitely worth the extra cost if you plan on using your Jeep in extreme off-road conditions. If swapping huge tires, adding a locker, or using a high-horsepower motor are in your plans, you may want to consider a heavy-duty slip-yoke elimination kit. These kits are not that difficult to install, so a Jeeper with some mechanical know-how should be fine doing the conversion himself. Most of the heavy-duty kits come with a new mainshaft, which is styled after the NP241 truck unit. This design does not have the tapered area where the speedometer gear meshes with the mainshaft and offers a 54 percent gain in strength over the stock shaft.

If you go either high-dollar or low-buck, you will need a new collapsible-type driveshaft. While you’re at it, you might as well get a CV-type shaft which should eliminate any further vibrations for a few more coins. Many companies now offer a custom-made driveshaft along with the yoke kit. Lift height is important for the length of the new driveshaft so be ready when you order. We ordered a JB Conversions heavy-duty slip-yoke eliminator kit, and a CV-style driveshaft from East County Driveshaft of El Cajon, CA.

Both parts should provide plenty of strength for our mildly lifted ’97 Cherokee, as well as get rid of that annoying vibration. We took a ride down to MIT (also in El Cajon), where Mitch Wilson helped us out with our installation and taught us some tricks along the way. Here are some tips that should help make your install go a bit smoother.