I'm guessing you have a 2020. For the 2021 model year, the "build" function on the LR website is incorrect. On the 2021 factory order system, you need to order the offroad pack to get active locking differentials. (And of course the advanced offroad capability pack to get terrain response 2) You can't order the torque vectoring/active diff separately anymore.
When I was picking up my new Defender yesterday, I learned that current delivery time is 4-6 months from order entry and they start taking orders for the 2022 model year in March.
Yeah we ordered ours over the summer and picked it up in October.
We were able to get everything (active differentials and terrain response 2 etc.) with the more road biased tires on black 19" wheels.
I think the tow package was connected to the active diffs or maybe it was just too the AT2. Like you said the configurator makes it look like you can still get that. But if the dealer order system won't accept that who knows.
Might still be worth someone trying if they order. Maybe it was a glitch.