I have a 4x4 91 ranger that is making a knocking noise under the floor?

I'd say if you are lucky you have a bad universal joint.But it really sounds like your ring and pinon gear are bad and it is trying to lock up that's in your front differential where the front drive shaft hooks up .Sometimes you can check it by shaking it and also pushing up and down there should be no more than maybe a eight inch play any more than that take to a repair center