THE WIRING ON THE XJ IS FINE. What I AM telling you is that you need to stop obsessing over the cluster, and just go find a new one. All XJ/MJ's with out tachometers still have all the stuff to run one built into the harness. What you will need on the other hand is the coolant temp sensor and the oil pressure sensor on the XJ motor, as right now they are set up for switches for the idiot lights.
THE '90 CLUSTER HAS A DIFFERENT PLUG NOT BECAUSE IT DOESN'T HAVE A TACH, BUT BECAUSE ITS FOR A '90, NOT AN '86.
Get a new cluster. '88-'90 should work.
Rob L.