So I changed my rear end oil this morning, and found it was foamy and tan, with rust on the cover inside. Is this normal?
I have a 2002 Fx4 Ranger, with the Torsen LS rear.

I also found a peice of metal, twisted up, and it looks like its sheetmetal or something. What do you guys think its from?

 

The fluid looks like water has been mixed with it. Did you run the truck through some deep water somewhere? Pull the axle shafts and check the seals at the outer ends of the axle tube. At least one of them allowed water to get into the axle housing. Both tubes, the differential housing and internal parts need to be flushed thoroughly to remove all traces of water and the contaminated fluid before refilling with new fluid.

Oh yes, you shoud also check the pinion seal as well.

Sorry, I can't help you on the metal piece.

 

There is usually a vent on the top of the housing to allow the differential to "breathe". If you have ever had it completely submerged in water, that could be the source.

 

I would also check the bearings close to make sure that they are not pitted.

 

Definitely not normal.! It appears that the axle has been submerged in water and water got in either thru the seals or the differential vent.

Do what Michigan66 said and while you have the the axles out wipe everything down, inside & out, to get that contamination off everything. Afterwards, you are going to need to drive 100 miles and change the oil again to flush out the rest that chocolate mousse (and maybe a third time).

I don't know about that sheet metal part, the photo is pretty fuzzy, but it reminds me of one of those sheet metal parts used under an array of bolts where you bend up the tab to lock the bolt (most likely under bearing cap bolts or the ring gear bolts).

Trouble is, I looked on an exploded diagram of the rear end and I didn't see one of those. That doesn't necessarily mean that one isn't used. Examine everything in the rear end to see if you can find where that came from. Needless to say, it would be bad to have any more parts floating around. And you don't want loose bolts either. You're lucky that the one you found didn't get caught between the gears and completely lock up the rear gears or literally explode the whole thing apart.

In case you don't know, since you have the FX4 and the Torsen L/S differential, for oil changes you MUST use a synthetic 75W-140 oil. You'll be lucky if that contaminated oil hasn't ruined all your bearings.