Ok, this morning I put the car back up on the jack in my garage again. This time I completely removed the inner drivers side engine cradle mount (referred to as the #2 mount by GM). I then put the whole assembly back together and torqued all of the bolts to the factory specified 141 ft-lbs. As soon as I started lowering the vehicle off of the jack stands I knew I had the problem nailed, because with the mount in place it would have popped and groaned as it was being lowered. I then VERY cautiously drove the car around the block for a while without the mount. There are actually sounds that I used to hear in the car that I didn't even know were related to this mount until I drove it today, THE CLUNK IS GONE!!! Now I have a new mount assembly (all components of the #2 mount) on order with my dealer and I intend to install it as soon as it comes in on Tuesday. Unfortunately I should mention that there is still some chance that the problem is actually with the frame itself. If the hole in the frame has become badly erroded the new mount will not press fit into the hole as it should and it will probably clunk again. But for now I am being cautiously optimistic that its simply an old mount. I'll keep posting until I have the problem licked, once again I hope this makes it to someone who has the same problem.