[JerryG]

Andy,
I've been there and continue the struggle to escape getting stuck behind and under. It has to be the polio of golf.
Your reference to extensor action is the beginning of the cure. Daryl, EdZ, O.B., KevCarter and many others will hopefully chime in very shortly.
Part of the problem may also be attributed to bowing and rolling the left wrist as you take the club back. That stupid move will influence the move to go back and in rather than up and in. I believe the bowing is a precursor to getting it too inside. Instead of that move focus on the cocking in the Snoose Box and make sure that is operating instead of the left wrist bowing. Cocking the wrist up in the move back will, in my opinion, influence you to take the club up instead of in. The right hand, in the act of extensor action, will help with these new feels and you will be on your way. Practice very slowly for lots of reps and don't bother with a ball until you are at a point where you think it is feeling more natural.
Now, because this is my bane as well, I cannot wait for the other folks to chime in.