it is part of the crank position sensor, look at your current96 motor you will see what i am talking about.

 

My 96 bronco is mass air and obd II and motor is a speed density system and obd I. Is there a major difference between wiring harness? I am thinking that I need to install mass air meter onto 91 motor and thats it, am I wrong?
Thanks.

 

yes you are correct you will use everything from the current 96 motor onto the 91 including the harness.

the 96 obd-II system gets timing from the crank position sensor that reads off of a star shaped tone ring behind the main crank pully. all of this should be visible looking down at the crank pully on your 96.

Also take note when you pull your current 96 motor out to see if there is any crossover emissions stuff on the back of the heads. the 91 model motor would have holes in the back of the heads for a crossover tube.
with both motors on engine stand i think you will figure out what goes where and such.