Well, I just asked about it last month, and here is a new policy
As of June 9, DL no longer accept pets/animals as cargo on all 76x planes.

You may read about it more here:
http://www.flyertalk.com/forum/delta...-9-2012-a.html

The question pretains to both:
(a) Is there a reason for the change? (structural wise?) were some changes implemented on the 76x planes that does not allow flying pets anymore?
(b) As noted in the other thread- the change seems to apply to the 767-400 series as well. I also assumed that the structure of the compartment in that category is better planned for animals transport. So, I was caught 'off guard'.

Were you informed of specific reasons/technical obstacles leading to the change?

(side note- I presume that the 75x are still accepting pets/animals as cargo, within the current limits)

Looking forward for the trip report!