Is one bottle enough to fill it?
No, I used almost 2 on the front diff, although that was with some spillage. The rear, I think, takes more. I'd be tempted to get 4 quarts to do front & rear, that SHOULD be enough.
The frustrating thing is, the manual gives 3 different volumes; US Imperial, Imperial and Metric. If you run each number through a converter, it'll give you different volumes. I never found a 100% straightforward answer aside from 'about' or 'approximately'.
I also did a lot of research and unless you plan to tow or heavily offroad, you can stick to 75W-90. The 75W-140 synthetic approaches double the price. The manuals recommend a 80w-90 conventional oil for front & rear. The 140 stuff could hurt fuel economy since, when warm, it's much thicker. Some people swear by Royal Purple or Amsoil, personally, I'd stick with Valvoline or Mobil 1. I try to stay away from the Walmart or other re-bottled stuff (auto parts stores) since you don't always know what it is. I picked up 3 bottles of Mobil 1 75W-90 Synthetic for $7.20 per quart shipped at Advance Auto Parts with a coupon, cheap.
As FR-425 said, put the Motorcraft limited slip additive in if you have a D4 rear. Even if you get gear oil that is rated for limited slip, put it in. It's not that expensive and it saves you hassle down the road.
As for the ICM, that should probably be your last effort. Places like RockAuto or Advance Auto Parts likely have it for cheaper and you can easily find coupons for them.
When the ICM goes into limp mode, does that mean that the timing is no longer computer controlled and set at the default 10 degrees BTDC? If so, you could either check the timing with a light (hard to do since there's very little in terms of reference marks) or yank the SPOUT connector and see if your MPG changes. If my theory is correct, when you pull the SPOUT connector, that will make timing go to 10 BTDC and should hurt your gas mileage. If it doesn't, then you know that the computer isn't advancing the timing at all (with the SPOUT) and something is wrong.