I had this problem when I swapped my standard stock out for a VLTOR.
I was able to easily separate the stock/tube assembly from the lower receiver, but I had problems separating the buffer tube from the stock.
Since I didn't want to take a big wrench to the buffer tube and screw up the threads, here's what I did:
First, I put some CLP on the big screw on the back of the buttstock, and let it set for a few mintues.
Then, I reinstalled the stock/tube assembly back onto my lower receiver.
Then I put the front of the lower receiver on the ground (with the buttstock facing straight up) and applied downward pressure on both the receiver and the screwdriver when I tried to unscrew the buttstock from the buffer tube. I think I also held the buttstock/receiver joint so that it wouldn't unscrew.
That worked for me.