Here are your options to change your speedometer. In newer fuel injected Jeeps the speedometer works off an electric speed sensor. On the Eagle it works mechanically.
I just found my new MPI (Multi Port Fuel Injection) speed sensor on the evil bay for 28.03 (buy it now, more than 10 available, free shipping). In comparision Autozone wants 70 dollars and Hesco wants 122. Helpful search terms to find what you want include the words "new" "speed" "sensor" "transmission" "vehicle" "Dodge" and "Dakota".
The same one was used on 88-93 Dodge Dakotas as well as 91 to 93 YJ's. The Speed Sensor has an input and an output. It screws into the CJ style transfer case adaptor that is also used in a 1992 XJ. The XJ speed sensor is nearly identical except that it doesn't have the threads and hole for the mechanical speedometer.
The first picture shows the YJ speed sensor I'm talking about. The second pic, in the middle, shows the 92 XJ version that does not have the hole or threads in the center (to be honest it shows a Dodge speed sensor that’s nearly identical). Later XJ’s replaced this two piece design with a one piece electronic only design. The fourth picture shows the entire two piece design that gets installed into your transfer case.
Option 1: Mount it directly on the back of your Speedo using a brass adaptor to go from tiny threads to big threads. It’s tight but this keeps everything else stock. “Tougeagle” used this method in his popular write-up.
Option 2: Mount it on the firewall, have original speedo cable go to it, then an International Scout upper speedometer cable which is only about 2 feet long go from that to your speedo cluster. Scouts had some doohicky on the firewall that interrupted the speedo cable just like this does.
Option 3: Get a common 92 XJ adaptor that plugs into the side of the transfer case. You'll need that piece from a junkyard. Thread this 91 YJ speed sensor into it. Get a 91 YJ mechanical speedo cable (threaded at both ends) to go from your dash to the speed sensor on your transfer case.
Option 4: Combine option 2 and 3. This is my choice and my recommendation. You use a YJ mechanical speedometer cable instead of the original Eagle Speedometer cable. Screw one end of the speedo cable directly into the XJ transfer case adaptor and the other end into the speed sensor mounted in a convenient spot under the hood. I’ll have the short Scout cable to go from the speed sensor to my dash. This keeps all electronic junk organized inside the hood.
Option 5: Eliminate the mechanical speedometer totally by modifying an XJ speedo to fit into the Eagle dash. I included a picture of the dash I’m making for my main car with an XJ tachometer and Speedo.
The last three options are the best in my opinion because you can just unscrew the speedo cable and leave the transfer case fluid alone if you’re just removing the transmission to work on something else. It’s a mess to deal with popping that stupid integrated speedo cable out of the transfer case all the time. All Jeep transfer cases are interchangeable if you upgrade later, so you’ll only have to buy these parts once.
The last picture shows a 242 transfer case. The threads are visible along the slip yoke housing. That's where you'll find the internal adaptor you need from a junkyard. The speed sensor that screws into it and the different size gears are all available new. All early XJ's had these but eventually they went to the one piece electronic only design. A CJ Dana 300 piece is supposed to be the same, but I haven't confirmed this. I just know all the new process transfer cases are interchangeable speedometer adaptors.