The stadium might have been sold out in that period, but I can tell you that as the president of the Bills Backers chapter here, I had more free tickets given to me for the Bills games at Foxboro over those years than I could give away to our members.
In 2000/2001, season ticket holders were clinging to their seats because they didn't want to lose choice seats in the new stadium, which was under construction.
Boy, did I enjoy coming into my office on Monday :-)