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There should be a tag on one of the rear end housing bolts with some coded information. If you could check that and post the numbers, I'm sure someone could decipher it for you.
There were various gear ratio's available. One quick and reasoably accurate check is to count the wheel revolutions and compare them to the drive shaft revolutions. Four revolutions of the tire vs. one revolution of the driveshaft would be a 4 to 1 ratio.
The most accurate method would be of course to count the teeth on the ring gear and divide it by the number of teeth on the pinion gear.....
Dave is right about the tag, get the numbers and post them up and we could tell you if you have a Limited slip or not and what gear.
RTM is right, I've never seen a F-150 4x4 come with anything other than 3.50 gears.
Yes a full time 4x4 may more than likely have the 3.00 gears and I have read that it was more common to find 3.00 in full time 4x4's than than finding 3.50 gears. I do have a 78 bronco with 3.50 gears and it is a full time set up. I haven't seen to many full time set ups running around either so they are even hard to find or they are around here.