Milky Gray Differential Fluid

Change it immediately. It is water in the gear oil

 

I do find it odd that both diffs are milky colour.

As stated above, most likely was the diffs under water?

 

2 feet is already close to the vents on the housing or above that level.

 

Depending on the year of the truck. I think but I am not sure, but trucks manufactured after 1997 are synthetic gear oil. There should be a round tab around the oil fill nut that tells you what it takes. But I would recommend synthetic. You can call Peterbuilt give them the last six of your vin they will tell you

 

Change the tranny fluid too. I've heard that about automatic transmissions in cars or pick-ups but don't think it applies to manual. 50wt synthetic in trans and 70wt synthetic in rears I think

 

Friendly word of advice - clean fluid is ALWAYS better than contaminated fluid. Especially in something like a transmission.

You're going to want to find out where that water ingressed from or you could potentially waste $$$ on fresh fluid. Could be from a faulty breather. I would also make dang sure to inspect the top side of the diff housings. If trucks aren't washed often I've seen them rot right through. Usually under/around the torque rod pedestals or where the suspension attaches to.

 

Water... You got water in your differentials. Drain it and check all your seals... Might even have to split your diffs to reseal the 2 halves.