Rear end (differential) locked up?

Raise both rear tires off the ground and try to spin each one by hand. It's very rare for a differential to lock up, especially without some kind of warning. Rather than list all the possibilities of what can cause trouble it's easier to describe what you should find. If you have the more common "open" differential, when you spin one wheel forward the other one will turn backward when you have the transmission in park. If you shift it out of park and turn a wheel, either the other wheel will turn the other way or the drive shaft will turn depending on which one has less friction. The drive shaft will turn easier in the forward direction than backward.

If you have a locking differential both wheels will only turn the same direction and they'll force the drive shaft to turn so it can't be in park. In park neither wheel will turn continuously but due to normal play either wheel will be able to be turned an inch or two. That should be enough to verify a brake is not locked up.

You still haven't listed the symptom. The first part of the differential consists of four small gears all with 45 degree beveled teeth that form a square. Two are stationary and one is splined to each axle shaft. That's what causes one axle to turn one way and the other turns the other way. We know that part is okay now.

That entire assembly is placed inside a housing that is free to spin on a pair of really beefy bearings. When it is turned by the drive shaft that housing tries to turn both axles but either one is free to turn faster or slower than the other one when you go around a corner or spin on ice. That housing is what could be locked up, either due to broken teeth on the large ring gear around it or debris stuck between the teeth, or from a bearing failure. All of those things would be very unusual.

Now that it's jacked up, try running it in gear to see if the drive shaft will turn. If it will not, unbolt the rear universal joint from the axle and pry it forward to remove it. Watch the bearing caps so they don't fall off or get pulled off. Once it's removed try to rotate that yoke by hand. If it turns freely there's something broken inside the transmission or transfer case. If you can't turn it, you'll have to remove the rear cover to investigate further. If the transmission is not in park you should be able to turn the drive shaft by hand. It will turn pretty hard one way and not quite as hard the other way.