Hi. New to the forum with an '05 XC90 V8 @ 185k miles, which is currently only FWD (no power, or almost none, to rear wheels). Figuring this is not too unusual, I've searched for a thread or FAQ on potential causes, ways to diagnose, and repair. If anybody can point me to something along those lines, I can probably avoid a rehash of a subject that's already been covered one or more timesf.

OTOH, if anybody wants to jump right in on my problem, here's what I know so far:
Problem dates back to some time last winter. It wasn't evident in normal use because I still had good traction from my snow tires, but when negotiating 8-10" of snow I realized the rears weren't helping at all. Subsequent throttle-mashes on packed snow and ice confirmed it. Asked a local tech to check it while he was working on a different issue - he found no codes from the "rear controller" (which I presume is the DEM?) and no "drivability issues", but admitted that he hadn't actually put it up and in gear while on the hoist or done anything else beyond checking the codes and taking it for a drive.

So, while switching back to snow tires this week, I put all four corners up on stands and started it (yeah, I know, maybe not the safest technique, but I don't have a hoist). Found that in park or neutral, both rears spin pretty freely (and in neutral, the fronts spin slowly and can be stopped with one finger - no idea if that's normal or not). In D or R, the fronts spin at a reasonable idle in the direction you'd expect, the left rear is completely lifeless (spins free by hand), and the right rear wants to spin but can't quite do it - if I spin it by hand in the proper direction (forwards in D, back in R), it's clearly getting a little torque in the same direction because it'll spin for a revolution or three on its own before slowly coming to a stop; it's reluctant to spin counter to the front wheels (but can still be spun that way by hand), and stops almost immediately. Driveshaft spins in D,N&R, and I couldn't spin it by hand in Park or with the motor off (couldn't get a real good grip on it).

Any help, comments, abuse or advice will be appreciated. Hoping this is something that I can fix on my own, or at least to be better armed when I take it to a pro.