I modified an R-12 Freon tank (4 gallon?) several years ago with a dip-tube, which doubles as a filler hole. I added an air fitting to the tank to allow you to pressurize. On the delivery end is 10' of hose with a ball valve and a narrow steel filler tube that fits inside diffs or transmissions.
To use this, you pressurize the Freon tank to <20 psi, crawl under your truck, insert the tube into the diff or transmission, open the ball valve, and 90wt flows quickly into the cavity. Very slick and low-effort. Downside is that once it's full of 90wt. you cannot easily change over to ATF or a different oil.