I'm trying to identify the rear that came in my mustang. Its definitely a 9" as identified by the 2 bolts being covered near the bottom.

I did the test spinning just one wheel, 3.5:1 looks to be the ratio.
However, when I keep the yoke from spinning I can't spin either of the two hubs. I'm doing this on the bench, I guess I need a longer bar. Will head out and keep trying.

Is 3.5 in a 66 with a warmed over 203 and an AOD a good ratio?

I spoke with the PO owner last night, he "thinks" the rear came out of a '77 Mercury Monarch.

 

OK so more questions.
From what I could find, it says that posi is the number of turns on the wheel to determine the ratio but open you need to double the number. Is that backwards? I got the rear to break lose with the yoke not turning and it looks like this is an open rear

 

Your lack of turning indicates a Traction-Lok. Not a "Posi", which is a GM item. Both are "limited slip differentials".

3.50 is an excellent ration for all-around driving, and the AOD overdrive will give good highway mileage, too.

 

fully agreed. hold the yoke and spin one wheel (axle). if the other axle turns the opposite way (one wheel turns forward, the other backward), then it's an open rear end.

 

I just about have the rear apart. Axles are out just letting the oil drain off before I pull the 3rd member out. Then I'll know for sure what I have....well I'll be able to post pics then someone can let me know exactly what I have

 

Here is what I found. Little more info. If I put the shaft into the right side (of pic) and lock down the yoke I can't spin the shaft. Maybe if it was on the car and I would have enough leverage to make it spin. The shaft on the left side causes the shaft on the right to spin backwards. Does this mean I have the traction lock?

 

There was a lot of RTV on both bearings. While I'm in here should I replace them or just repack them or do nothing. While its apart I hate not just replacing all the seals and bearings. Not knowing what this is I'm not sure how to get parts. This is my first time opening a rear.

 

So here is what I was able to figure out. It looks like it did come out of the 77 Monarch and if I described the rotation correctly then 2+2gt's call on the traction lock is correct. Still not sure I described everything correctly. I also determined is a large bearing 9". I'm still digging for parts but I think I have enough to go on to at least reseal it and put new bearings in it. Was thinking about upgrading the breaks to this http://www.summitracing.com/parts/WI...odel|Ford+9+in. if it fits since the front is going to have 4 piston Wilwood brakes in it.