2 Piece Driveshaft and vibe issue help?

I have a 2 pc driveshaft, when I drive low I do get vibe, but when I let off the gas or slow down I get some rattling in the driveline.  Not sure about the rattling, is this because it is that far off?  I didnt set it up, just trying to fix it for a friend.  Anyone have any insight?  I did some research, spacing the carrier up is out of the question, because it is at the bottom of the tunnel, not alot of room, but I think I need to adjust the rear end.  I will get some pics up in the morning.  The lower I go, the worse it gets obviously, raise it up and it almost goes away.  Havent had much of a chance to diag it yet, just tryiing to get an idea.  It is a 2 link rear setup.  Thanks!