2000 Ford Expedition Did I lose the rear end

Drive Train Axles Bearings problem
2000 Ford Expedition V8 Two Wheel Drive Automatic 170k miles

Wife was driving normally than Parked. Attempted to reverse and she couldn't. She was able to (still is) drive forward with a large amount of resistance. Reverse is shot. It feels like the E-brake is locked (but it's not). She managed to drive home (crying on the phone to me) and with it in park, it doesn't move. In neautral is moves forward (with a lot of force) and not backwards. I am thinking it's the rear end. Oil change place told her she had a bad seal and was leaking diff fluid. He "added" more, that was about 3 months ago. I noted minimal crud buildup on the RR inner rim. Minimal at best. It should also be noted that fluid leaks at the tranny yoke (and has for a few years) ATF is constantly checked, and is just barley on the low side at the moment (still in the Xross section of the DS).

No towing, average city driving. What tests can I do to determine what I am dealing with? I am very mechanically inclined, worked as a mechanic about 15 yrs ago. Have about half those tools.

If I do need to rebuild the pumpkin / bearings / seals, do I just pay to have a shop do it or is this doable for a driveway repair.

***EDIT***

Typical for the month I've had.
After typing this, I went out to the truck to MAKE SURE the P brake was not set and that it is functioning correctly. Mind you the car is facing forwards in our driveway with no reverse. So with the engine off I put it in neutral and it moved normally (driveway is flat). Started it up, let it run for a few and put it in reverse, it starte backing up just fine. Back to drive and it's working. There was never / or is any whine from the rear end. Took it around the block and I noted that in drive it does feel like there is some kind of drag going on. Reverse worked just fine for the few minutes I was testing this. With the engine warmed up as well as the tranny, I checked the fluid again, it was at the bottom of the Xross hatches. I let it idle for another 5-10 minutes and the fluid was between zero and the bottom of the Xross hatches. After I saw that, I shut it down. Added about 1/3 qt. Of Motor Craft ATF and we'll see. I think I may be the dumbest former mechanic out there. At least I hope so. Just wanted to update this thread incase a real mechanic was about to spend a bunch of time on what I previously stated.

So what causes this additional "drag" and losing reverse? I can't recall if the tranny has ever been flushed. Sigh.