Quick and dirty parts list to build a junkyard 231HD. I've seen a bit of conflicting information on this and other forums, so I will share what I have learned in piecing mine together. First off, there is no "231HD" case from any Jeep or Dodge vehicle that is simply plug and play. There may be other varyiations to this, but below is what I know will work for your WJ. You can either piece it together yourself, or buy a pre-made, brand new case from JB conversions that includes the items listed below:
1. 231D out of a '98 - '01 Dodge 1500 pickup. This will include the 1.25" wide chain (231J's (jeep models) are 1"). This case will also include the updated and stronger aluminum shift forks. Prices vary anywhere from $150+ depending on your region. Check www.car-parts.com
2. Source a 6 pinion planetary from a late 90's model dodge transfer case (i.e. 241D, 241DHD). Late model is critical so the gear cut will match. The 231D (and 231J's) includes a 3 pinion planetary that is rated at 600 ft/lbs torque. The 6 pinion is rated at 1,400 ft/lbs torque. I found several used on eBay varying in price from $75+. JB conversions can get you a new one for $200. I opted for used.
3. HD Slip yoke eliminator. JB conversions or Advance Adapters is your best bet here. The 231J/D SYE is the same. Includes a 32 spline output shaft, which nets you an increase of 54% torque capacity over stock output shaft. Can go super shorty or regular. For long wheelbase WJ's, there really is no reason to go super shorty. Regular SYE will run you $200. I opted for JB due to a couple of minor advantages that I perceived over AA. Both are very good, however.
4. Swap your input shaft over from your 242/247 and it will match perfectly to your trans.
a) No 231 bezel swaps over so you'll need to either do nothing or ghetto-fab some labels. The 231HD shift pattern is opposite from that of a 231J, as follows: 4L - N - 2 - 4PT.
b) You may have to modify the linkage arm, depending on your donor case -- there are a few good threads on this. Note
: you do NOT have to modify the linkage if you swap the linkage from your 242. A 231J or 247 linkage will not work. See pic of 242 linkage arm, which is direct swap.
c) You will also need a 1 1/4" freeze plug to plug the speedo port, which is a few dollars at any local parts store (speedo port is nice b/c if you consider an axle swap down the road, you can always run speedo off the transfer case and not the axles). Advance Adapters offers a billett plug with the 231j style horeshoe retainer. I opted for this over the freeze plug, which apparently AA was seeing some issues leakage-wise. See pic of billet plug/retainer.
d) Some 231d's use a vacuum switch for the dash indicator light. The 242 switch is a direct swap, but the detent is longer so the dash light is not accurate. Either leave it alone (I think you should know whether or not you're in 4wd or not
, or you can buy a 231J 4wd switch and swap it in by splicing the wire to the new plug. Threads are all the same.
e) Alternatively, you may be able to swap in a Dodge 241 (basically a 231HD) which has 6 pinion planetary, the wide chain, etc.., but the 241 SYE kits are much more pricey. Plus, I am not 100% sure if the input shafts are a direct swap.
Total should run you about $500 in parts. JB's 231HD is $1,275.
Hope that helps.
242 linkage (w/4wd switch in background) + speedo housing plug pics: