2004 Crown Vic, was driving from Texas to Utah and didn't realize the differential cover plate had developed a leak enroute. I was trucking along at interstate highway speeds the whole time. I did hear a faint sound like I was driving on a zipper but I thought it was tire noise from the unfamiliar roadway I was on. Just about almost to my destination I could hear that louder zipper sound, which I guess is gear and another low pitch moan/grumble....bearing? After seeing some drips on the garage floor, I took the cover plate off and recovered about 3/4 of a quart of fluid. From what I researched online my car should hold about 3 quarts. 

Question 1: Why does nobody create and install a sensor to tell you when the differential fluid is getting low? Considering its at the far back of the car you have no way to know when you sprung a leak. 

Question 2: How bad do you think the differential and axle damage is? Am I looking at having to have a shop do a full differential and axle rebuild?