In this video, I show the steps needed to change the gear oil in the rear differential of your Ford transit 350XLT. This particular axle is not a limited slip axle. Therefore, you do not need the friction modifier. The proper weight differential fluid.
Is 75 /85 from motocraft and its full synthetic. However, I used valvoline 75/90 full synthetic with the friction modifier added. Valvoline doesn't sell the gear oil without the friction modifier atleast in the pouches. I've never had any issues using gear oil with limited slip additives already added to the oil in non limited slip axles. However, your experience could be different
You should always follow what your vehicle manufacturers service book. For this 2017 Transit 350 XLT single rear wheel van, the book called for 3 quarts of Motorcraft 75/85 full synthetic without the friction modifier.

If you are unsure of what oil to use, pull the differential cover off and look for the limited slip clutch or electronics.