To fill both differential and manny tranny, I made up a fill hose--about 6' of clear plastic hose (maybe 1/2 id). In one end of it, I stuck a small funnel.
For the diff, I put the funnel by the diesel-fuel fill and run the hose over the top of the tire/wheel and down to the differential fill plug. Pour warm gear lube in the funnel and it runs into the differential.
For the manny tranny, I put the funnel in the engine compartment and run the hose down to the tranny fill hole. Pour the Mobil1 ATF into the funnel and it runs into the tranny.
I made up this device after generally making a mess trying to fill with the squeeze-the-bottle technique and wasting money on a $10 Pep Boys filling device.