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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.
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Tuesday, March 26th, 2013 AT 7:23 AM