submerged differential

LOL!

It makes sense.

The bellows idea is great, but considering it needs to handle expansion/contraction over a temperature range from below freezing to perhaps pushing 200 degrees on a hot day under heavy towing loads...

That's a pretty small bellows given the amount of expansion of the air in the housing over that temperature range.

And if it was installed on a cold day, on a cold diff, the bellows is already pretty well expanded and has even less room to grow with heat.

 

Gen1 has the same breather on the rear diff, just checked, same placement. I can't check the fluid right now since it's on a slope. Last time I replaced the oil was 50k ago with Amsoil gear lube, it was clear and fine then, and it had been submerged crossing streams. Is a mod even necessary???

 

Not necessary no. Good idea though.

Chances are your breather will close as you go through the stream and nothing will get sucked past your seals and everything will be cool but for the cost of the hose and a fitting and a couple hose clamps... why bother taking the chance. It's a lot easier than checking your gear oil for contamination after every trip and a whole lot cheaper than replacing your rear diff because you got water in the oil and killed your 3rd.

 

Seriously, it is such an easy mod. See my link for all photos. Do not reuse the stock vent. That defeats the whole mod. You need a two way vent actually a breather, at the end. Mud gets in past axle seals, not past stock vent.

 

Just did a quick calculation. From 0F to 220F causes a 48% air volume increase. To accommodate this volume change without pressure buildup would require a much larger bellows.

To the OP: scratch the Creeper Breather idea and do this instead:
http://www.tacomaworld.com/forum/2nd-gen-tacomas/15686-diff-breather-mod-dummies-pics.html

 

If water getting in was such a big deal, I would have been screwed by now. LOL I have driven for 40 plus minutes and then crossed ~3'(water was flowing over my hood generously going against the current) of water to get to the other side of the creek, went a couple miles through the woods to my hog trap, turned around and repeated the crossing. I've done that probably 30-40 times along with numerous cold crossings while working on that particular property. Not to say that it cant and wont happen and that this mod is not a good idea, but as long as all your stuff is in good shape, I think the chances are slim due to my experiences.

Having said that, I do plan on relocating my diff breather.

 

You should check your diff fluid for water

 

I just replaced all my fluids after 90k miles. Front diff looked the worst.