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.
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