Please consider returning your 75w-90 gear oil and getting the 75W-140 gear oil that Ford recommends. When compared with motor oils, 90 gear is about the same as 50 motor, and that is too thin when your rear end gets hot on a long summer trip. The Mobile 1 synthetic that I bought cost me $19 per quart, and you will need two quarts and a nice pump. You don't want to spill this stuff, as it is very pricy.
An open differential will send traction to the wheel with the least traction when it is slipping.
Yes, I did pay too much for the TrueTrac, but oh well.

 

Not really worth returning the oil (ordered it with a whole bunch of other stuff). I will have to go and buy some more. How does this sound: Use it for now (winter time) and after the break in period, go and change out the oil to the heavier weight?

But you did get a great deal on the install... It all evens out.

 

I think that the 75W-90 Gear Oil would be safe for 500 winter miles, especially if you aren't doing a long trip wherein the differential oil would get hot. I would then change it out. Sounds like a good idea.