I know this has been covered before but I just completed it and wanted to post pictures in case it might help someone out.
I picked up a used 231 out of a 2003 TJ with 18000 miles. The ZJ I have is a 1994 4.0 and using the 249. I also am running the Ford 8.8 rear diff with disk brakes. I have a 6” lift so I wanted to use an Advanced Adapters SYE kit for a couple of reasons. Not just for the drive line angles but I also did not want a slip yoke that sucks dirt in the seal all the time and causes it to leak. The SYE kit solves that problem because it has a flange and is also over 4” shorter with the kit installed. When I picked up this ZJ it had a mix of Tera, RE, and JKS control arms. I really wanted to move to all RE stuff along with the Mono Tube shocks. I have used this setup on other jeeps and it works great. The Tera and the JKS bushings were shot so it was a great time for an upgrade anyway. I went down to All J Products in Colton CA and they had everything I needed to get this job done. They also use the J.E. Reel driveshaft. And Quinn was able to find the 231 tcase for me. We pulled the 249 tcase out and by chance it had the same length input shaft on both the 231 and the 249. Those shafts have to be the same. The 249 input shaft will fit in the 231 case but you have to be sure of the right gear cut and also the spline count on the input shaft. There seems to be an older 21 count spline and the newer 23. The 249 and 231 I had both had the 23 spline count. Also the shift lever is different from both of these cases. You have to use the right one…Be sure to use the lever that came with the tcase. The short one if the 231 and the longer one is the 249. You can not use the longer lever from the 249 on the 231… it will fit but will not allow the 231 to shift right. Pulled the 249 out…installed the SYE kit in the 231…installed the 231 in the ZJ. Measured for the new drive shaft and called it in . Had it the same day and installed it. Filled the case, ordered the new shift pattern plate from the dealer and everything worked great. Set the new pinion angle in the rear and took it for a test drive…no vibes at all. I know this has been covered a few times but just wanted to give an update on the swap. Was a very easy swap to do and had no problem at all. Later...Clint

looks like you didn't have too many issues
Why did you opt for the 231 and not the 242 (which is also a popular swap for the 249)?
just wondering .......

Even though the 242 is full time...i really wanted the real wheel drive of the 231. The 242 does not really lock the front and rear together. For the guys that are in snow and ice alot...the 242 is outstanding. I live in So-Cal and we are always in rock so I really wanted it to lock both front and rear together. Our ZJ gets wheeled hard. Hope that helps. later...Clint

actually the 242 does lock the front and rear together, but it also has the option to not. 2hi, 4full time(unlocked) 4part time(locked) neutral 4low(locked) its what i have in mine, only problem with the 242 is the lack of aftermarket support. you made the right choice IMO going with the 231. im swapping in a 231 on my ZJ since my 242 blew up, and now i will be able to run a SYE.