Does it only do this when letting out the clutch in first gear or does it do it other times? does it make noise or just feel it chatter the whole car? It could be many things, probably going to need more information. It could be a clutch/flywheel chatter, it could come from the rear, it could even come from a rear wheel bearing maybe even a tight brake. Someone else will chime in and offer other more information. Best of luck in finding it.
Rodney

1st suspect would be clutch itself, including the disc, pressure plate, and/or flywheel. Could also be a worn/missing pilot bushing. Oil on the clutch disc from either the engine, or the trans, will create a chatter. A worn disc, glazed flywheel/pressure plate would too.
Does the chatter feel like it is from the rear area of the car, or more toward the front? Posi clutch chatter usually only happens when turning, but if the posi clutches are really worn, they could possibly chatter in a straight line.

If you have the old gear set out loose, look around the outer edge of the ring gear. You should see some numbers engraved on it. That will give the the ratio of that gear set. You'll see two numbers separated by a slash....for a 4.10, for instance, you might see a 10/41, the 10 being the pinion tooth count, and the 41 being the ring gear tooth count. Or....you could just count the teeth, then divide the ring gear by the pinion gear, and that is the ratio. Remember to round up...





|UPDATED|11/20/2012 10:59:31 AM (AZT)|/UPDATED|

It only appears when you first start off from a stop. The other gears are fine and it shifts smooth.

You could add a bottle or two of positrack additive to the differential, drive in figure 8s a few times, and see if that makes ANY difference at all in the chattering. If it does not, then I would say you're looking at a clutch issue.