To the best of my knowledge ford never made a factory true "posi" rear end. It shoudl either be a limited slip or an open differential. All you have to do is get both sides off the ground and turn one side. If the other side goes in the same direction it is either a factory limited slip or possibly has a spool installed or the spider gears welded. if the other side turns the opposite direction it is an open differential or a limited slip with worn out clutches. As for the gear ratio, turn one wheel, one full round and count the number of times the pinion shaft (where the drive shaft bolt on) turns. That will get you close enough you can probably figure out what gear it is. Do you know what rear end it is? 8", 9", 8.7, 7.5, 8.8? post a picture that will prob help too. Also if the tag is missing someone has most likely been inside of the differential at some point and left it off. Hope this helps some. I'm sure someone more knowledgeable can give you something that makes more sense. Best of luck to you.
By the way welcome to the forum