I received the replacement Tamron 17-50 VC and found it focused identically with the previous lens I got. I didn't know what to make of it, given that my 28-75 was focusing fine. I suppose it shows good quality control if the samples were all identical. I just wasn't sure why it was back-focusing.
Anyway, as I was getting ready to send the lens to Tamron for adjustment, I started thinking about the D300's lens adjustment feature. It would be nice to have that feature and avoid having to send back lenses. Just then, as if my prayers were heard, I found a used D300 on sale on craigslist in the very same city where we live. The price almost seemed too good to be true, but thought I'd check it out anyway. I didn't find any problems with it except for a small scuff on one corner, a missing LCD protector and a missing cover for the terminal to connect it with a battery grip (which are easy to find replacements for). So I bought it (with the thought of selling the D80 and the 28-75 to pay for most of it).
Surprisingly, the 17-50 VC focused perfectly well on the D300. Even more strange, the 28-75 focused perfectly on the D300 as well. Neither one required using the lens adjustment feature of the D300. I will be posting my impressions on both the 17-50 VC and the D300 when I get to use them some more.