this is a mount for the rear dif, it is liquid filled & similar to a liquid filled motor mount... the mount is not available seperately & is part of the whole rear subframe. part of the reason is that once the rear crossmember assy is all welded togeather it would be very difficult to try & press this mount in & out so it is not serviced seperately.
there have been a few cases of this problem popping up, think the first one I seen was viets but I'm sure he broke his with all the twin turbo launches & beating it hard drifting & stuff. I'm not too sure how bad it will feel when it ruptures & the fluid comes out. most likely it will transfer some vibration to the subframe but then the subframe is also mounted with isolation bushings so it may not even be so obvious, may feel a little looseness in the drivetrain as it allows the rear diff to move a little more than it should... I wouldn't think it to be detrimental to continue to drive with this mount ruptured & I really don't havew a better alternative to replacing the subfame yet as I haven't broken mine yet but I'm sure it's only a matter of time...
when I do break mine I'm sure I will not replace the whole subframe as this is going to be a weak point anyway if you really abuse your car... a better alternative may be to make a urethane mount instead, can't really say all that much as to what would be involved to actually do this yet as I haven't done it but ~$1,400 is a pretty big budget to come up with a better alternative I'd think...
EDIT: I now see viets response. do you know if they make anything other than a solid aluminum mount? I know I did a 350z with solid mounts & it really is much harsher with totally solid mount...