That's the carrier bearing and rubber isolator, and it's a piece of crap. They're no fun to replace, expensive, and difficult to balance.
Save some headache and order a 1-piece driveshaft from Shaftmasters (
http://ad.nicoclub.com/phpads/...3DNSN ) for about what you'd pay for a new carrier bearing.
You'll save some weight and have a perfectly balanced driveshaft as well.