1995 318i rear wheels locked up

[singring]

Two days ago I reversed my automatic 381i from my garage onto the driveway, then washed off the dust, and thoroughly washed the wheels to remove brake dust. I then drove the car back into the driveway, and went to work..the car sat unused for 24 hours.
Yesterday, I started the car, put the tranny into reverse, and the car wouldn`t budge an inch...the back end rose up from the pressure against whatever was holding the car still. I tried putting the tranny into Drive, and the same thing happens. I then checked for any engine code...there were none. I turned off the engine, and jacked up the front end...both front wheels turn freely. I jacked up the rear onto jackstands, with both wheels off the garage floor, and the wheels are locked. There is about an eighth inch of rotation either way, until there is a `clunk`sound, but that is it. Removed the wheels and the brake pads are not pushing up against the rotors.
I had a dealer brake job 2 months ago, replaced rotors, pads and sensors...they also found I needed a spring replaced, but I can`t recall what it was for ( I`ll dig out my invoice when I get home today).
There is some light rust on the rotors, but nothing that would suggest a seizure.
Tomorrow I`m going to jack up the rear again, disconnect the driveshaft from the differential, string a rope across the underside of the car so it supports the driveshaft, start the car, put tranny into gear and see if shaft rotates...I am 99% positive the tranny is not the problem...there was no slipping, or noise. This car has never been raced, spun-out, donutted, or abused in any way, just normal day-to-day driving...there has never been any rear end noise at all. I did notice there is a 2-prong electrical connector on the rear of the differential...could mean there is some kind of sensor that may have failed, and is causing the lockup.

I have come to 4 conclusions:
1. The differential has failed, but like I said there has never been any suggestion of a problem, and car has never been abused.
2. I flooded the brakes somehow when washing the wheels, and the emergency brake pads have seized against the back of the rotors.
3. The replaced spring from the brake job has failed causing a lockup.
4. The tranny has failed, but I really don`t think so.

The car was fine one day sitting in the garage, then rear wheels locked up on there own somehow....WTF

Anyway, I would appreciate any advice before I have the car towed to the dealer...and sorry for the long post.

Thanks much

[singring]

Finally got the wheels moving, and took to dealer to check out...the parking brake cams were seized, so will get them replaced.
Thanks for everyone's help