Even though you are noticing more bounce in the rear, you should probably plan on replacing all four, as your fronts will require replacement soon anyway. There is consistancy in the wearing out of these shocks. Replacing them with Tokico Blues is the best choice, economically and performance wise (as you will find many testimonials in other posts in this forum). As Brian mentioned, Joe at Infiniti Parts USA (
http://www.infinitipartsusa.com , 1 888 216 5328 ) can give you a good price on them, plus, they are very reputable and will be there if there are any problems.
Costs for four struts range from $240 (eBay) to $300. You can buy them individually; you should also plan on purchasing the dust boots, which is another $80 or so. Installation will probably run you another $200 or so, depending on where you go.