I've got a 1997 Ford Expedition 5.4 with 200,000. So my rear differential leaks a few drops of fluid everytime I drive it, looks like it's coming from the pinion seal. I've just been filling it toping it off with fluid.
Anyway, yesterday I got in it and started driving, I noticed a howling noise, and when I turned my rear tires started locking up. It was like that for about 5 miles until I got home, no matter which way you turned the tires would lock up.
I went under and topped off the differential with fluid again, and I pushed my parking brake in and released it. Got back in the truck and drove, and it was fine. Any ideas on what happened? I'm just worried it will happen again if I don't look more into it. Thanks the for the help guys.

 

How much oil did you have to put in it?

 

welcome to FTE.
sounds like the rear differential is on it's way out. i would get it checked before it breaks and leaves you stranded on the road.