Could be 3.55 or 4.10. As far as I can tell, 4.56 wasn't an option in 86.
Jack up one of the rear tires and put the truck in neutral. Spin the free tire around twice and carefully count the revolutions of the driveshaft. If it goes around three and a half times, it's 3.55. If it goes around just a bit more the four times, it's 4.10. If it's something else, let us know. :D