From my understanding, changing rear diff / tcase fluids should be a simple drain and fill. But per Jack's transmission, they state you should fill them through the drain bolt rather than the fill bolt, as filling through fill bolt leaves a chance of underfilling. Is this really necessary? I'll be changing fluids this weekend, and while filling through the drain bolt sounds messy and counterintuitive from a design perspective, I'd rather be safe than sorry

I know there have been threads on this topic before but I want to see how people are going about it after all these years.

 

It's not necessary, it's just a full proof way to do it. Mitsubishi service techs likely fill from the fill hole. Jacks's instructions are for those who rush the process by not making sure the diff and TC are actually topped off and preventing "false fulls." The best way to do this IMO is to pump oil in slowly through the fill hole and monitoring how much oil is actually going in by measuring amounts before hand. After that you can keep pumping in extra oil moments before bolting up the fill bolt.

 

It's not necessary, but it makes it extremely quick and easy.