I don’t think my transfer case from the Durango has a speed sensor like the jeep had, I have a output speed sensor on the tranny but that’s all i can findCheck your wiring carefully if the Tach signal isn't present you get no RPM gauge.
If there is a problem with the vehicle speed sensor circuit, you will get no speedo as it is driven from the VSS unit. Grounds and power along with signal are important. Also if you have a gear driven VSS in the output of the transfer case, make sure it is meshed properly rotated to spin the sensor properly. If the mesh is far enough out, you may not have the sensor spinning in the tail housing. This will cause no RPM to indicate and also cause driveability problems because the ECM doesn't know the Jeep is moving.
Unless you have a bad ECM, the 1998 or 1999 will run the 1996-1999 and some very early 2000 TJ clusters assuming the wiring was done right and the speed sensor is spinning in the tail of the TC. Again double check over at Hotwire auto the Mopar swap grid to make sure you're dealing with the correct bus type and haven't accidentally tried to use the wrong type.