i have a 97 blazer i was drive on the highway a few days ago. it made a noise that started to shake it felt like a tire was flat. then the tow truck driver was attempting to put it on the truck both rear tire were not move freely. i can feel the truck engage into drive and reverse i don't believe its the ujoints or the transmission but that it could be the spider gear.

 

Usually a rear end will make a racket for awhile before they lock up like that. I had something similar happen to a company F-150 a few years back. Howled for weeks, but the boss didn't care. Finally locked up at a stop light late one Friday afternoon!

Pop the cover off the diff, and post pictures of the carnage!