Rear diff does have a breather, but there are usually no problems associated with it, hence no one talks about it. I believe it vents over to the driver side... perhaps up behind the fender?

 

The rear diff vent tube is routed alongside the fuel filler neck, and ends right behind the fuel door compartment, but is an open ended tube, so it getting clogged isn't much of a concern.

No additives are needed for the open diffs, just need to lube the gears and bearings.

As to the 90 vs 140, there isn't a huge benefit to one or the other, however with as heavy as these trucks are, I like the fact that it says Heavy Duty on the bottle. BG also refines the bases that go into the 140 a little more than the 90. However, my main reason for using the 140(not to say anything against it, other than it being slightly more difficult to pump in through a small hose) is availability. Even with a BG rep that visits every week, sourcing the 75/90 is difficult, and with no real down side to the 140, it is what I use.