You say you have a 91. That's a Chrysler HO. Mine is an 87 Renix. Will the sending unit wiring match?
No, the sending units are different and send a different polarity signal output to the fuel gauge, but both the 87 and 91 4.0 MJs use the same Bosch fuel pump from the factory.
Would the 91 sending unit work in an 87 if the gauge from the 91 was put in to replace the one in the 87 cluster?