The 1961 F-100 is long gone, but I have a 9" Ford "pumkin" for sale. I am going to get around to putting it on eBay, but until I do I'll let you guys have a shot at it. It is from a 1965 Mercury. At 75,000 miles I put in a brand new Gleason TorSen limited slip differential and brand new Timken brand bearings, using the original 3:00 ring & pinion. I used this for another 75,000 miles of highway towing with no problems until I rolled the vehicle and parted it out. This differential takes 28-spline axles. Once when towing, on a snow-covered road, this saved me from getting stuck. The rest of the time it never makes a sound and acts like an open differential. It does not need special additives and (within reason) never wears out like clutch-type units do. (I wish it would fit the Dana 70 in my F-350, but my loss is your gain.) I put over $500 into it over ten years ago and figure it is worth about half of that used.