I had a similar situation when I swapped out my 92 cluster with a 93 that came with a tach. I found an interesting article "Installing a 1992 Explorer instrument cluster into a 1993 Ranger". Since I was reversing this procedure...changing an 1993 Explorer Instrument Cluster into my 1992 2.3L 4cyl. I had to reverse the procedure. In order to make the tach work from the 6cyl Explorer, I followed the writer's comments and had to remove the Blk/Yel wire from Pin #9 and tape it out of the way. Pin #9 was found on the vertical wiring cluster behind the instrument panel.

Just a note...I wound up swapping the entire instrument panel and pulled the 2 plugs leaving 6" of wiring attached. Through this article, I replaced the wiring (working backwards since I was going from a 6cyl to a 4cyl OBD1).

Good Luck