97 Mountaineer Rear Differential Locking when Hot

[jimberg]

Wow, I haven't been here in a long time, but I suppose I should go back to one of the most helpful group of Ford enthusiasts on the planet when I need help.

I have a 97 Mercury Mountaineer that has an issue where the rear differential is locking after driving about 20 miles. It's fine when I first pull out of the driveway, but when I get to my destination I'd notice a drag on the rear wheels as I pull into a parking spot. I had my wife make some tight turns in a circle by my house when it was acting up and I could see the outside wheels skipping through the turn. I've replaced all of the fluids for the the front differential, transfer case and rear differential. I used Amsoil for my replacement fluids. I also added the Ford friction modifier for the rear.

Any ideas of what could be the cause? Do I need to switch to a fluid other than what is recommended?

Thanks for any help that can be offered.