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That's punishing it right there. I can feel the heat from the torque converter through my computer screen.
I can't answer your question without seeing the truck. Either it has a LS or holding the brakes is making both sides see the same resistance. Jack it up under the pumpkin and with the truck in park, turn a tire by hand. If the other one rolls the other way with very little effort, it's got a functioning differential. A limited slip looks like an open, but has this S-spring thing between the side gears.
Ford has called a limited slip a "locking differential" in the past to add to the confusion.
Dunno when they started, but the newer 31spline 8.8's are set up like a 9.75 without the big S spring.
If the weight is balanced and traction equally crappy it is possible to spin both tires, I have done it before.
I'm just wondering what is the difference between a posi-traction rearend and a rearend with a locker?![]()