I recently removed the A2 stock from my AR-15 Carbine and installed a post-ban CAR stock.  I put it on just fine and it fits snug.  When I "lock and load" the rifle the bolt is hitting the retainer (I believe that's what it's called, the metal piece that slips into the long spring that sits inside the stock itself).  When this happens, it pushes it back and the buffer retainer (which is holding in the stock spring) is popping out because it too is spring loaded and I can hear it rattling around inside the rifle.  I never had this problem before. Any ideas on how to fix this?

Make sure the CAR buffer tube is threaded right and just resting on the lip of the retaining pin, Then tighten the tube nut. and it should work fine.

I'm not sure if I read your post correctly...

The receiver extension for the CAR stock threads into the lower receiver just far enough to overlap the buffer retainer and prevent it from coming out.  It sounds like you need to screw the receiver extension in further.