Will a 95 dodge Dakota v8 ecm work on 91 Dakota?

No, a 1995 Dodge Dakota V8 ECM (Engine Control Module) will almost certainly not work on a 1991 Dodge Dakota. While they might both be Dakotas with V8s, the ECMs are very different due to changes in technology and emissions regulations between those model years. The 1995 system is likely OBD-II compliant, while the 1991 system is OBD-I. These are fundamentally different protocols, and the wiring harnesses, sensors, and other components are incompatible. Trying to use the wrong ECM will result in a vehicle that won't run correctly, if at all. You'll need a 1991 ECM for your 1991 Dakota.