You can easily go off road without the locking rear diff. You just will have to use the traction control a bit more. The center locking diff is able to evenly distribute power between front and rear, but if you loose grip in the front or rear then all the power for that side goes to the wheel that doesn't have as much grip. The TC will kick in and apply brakes to the wheel that is spinning to move power to the other wheel, and move you along.
The locking rear diff enables both rear wheels to move together at the same speed without using traction control. So you will be guaranteed to have the rear wheels moving and the front wheels using traction control to get at least one more tire in the game.
Most LR4's do not have the heavy duty package. I think most buyers are interested in the bling and wont have a clue about all the hardware. I think LR has done a pretty good job of maintaining these vehicles off road abilities, even as their true target demographic is not as interested (directly anyway, they definitely like the idea of it and the pedigree)
I think it is odd to not have a full size spare on all models, but a lot of things shock me about the price point and the options, like the non-xenon headlights.. ***.
Anyway, if you are ordering an LR4, go ahead and add Heavy Duty pack, because it is well worth it and will make the LR4 even more capable.
As Clarkson said in his review of the LR3: "Of course you might be thinking, why on earth are you doing this. I am never going to drive my car over a scottish mountain. Thing is though, this watch, goes to 1000 feet, and i'm never going to take it down, im never going to 1000 feet, but, it's nice to know its ok if I drop it in the sink"
https://www.youtube.com/watch?v=QunqcDxP3Mw