if I had the whiteline bushings does anyone know a reasonable price to get this completed? if a DIY would take 4-6 hours (someone who knows what they are doing but not a professional) would a shop be able to do it for like 2 hours labor?

No, its 4-6hrs

So the differential bushing (fluid filled upper) is necessary for operation of the car under normal DD conditions, or is it more necessary for more rigorous operation under duress?
I recall reading about replacing it and two others with harder bushings for less rear wheel "hop" during acceleration from a standing start.
I don't recall ever reading (my brief year here) of anyone replacing them because they noticed the cars normal drivability impaired because of it's being damaged. I think there is enough stuffing in there for some cushioning even when leaking, so I wouldn't panic, just drive it carefully and get it fixed soon. Don't think the rear axle/chunk is going to fall off the car with a busted bushing.

OP there are several DIY type guides on here from people who have replaced their own. Doesn't appear easy but changing some parts never is.

As far as I know, the diff bushing is necessary to maintain proper driveline geometry during acceleration. Nissan chose to use a stupid gel filled one because it would dampen vibrations; however during hard driving (maybe other reasons as well) the bushing will pop. This loss of fluid allows for even more movement of the diff which will most likely lead to greater wear and less traction dying acceleration.

Is it necessary to replace? Well if you drive slowly probably not, but in that case yours probably also isn't broken. So if you were able to break it it is probably best to just replace it and not worry about it again.

Yes there are multiple parts that get replaced and I recommend you do them all, but it you want you can just replace the diff bearing so that you don't increase the vibrations that much.