2005 Frontier 4X4 Rear Drive Shaft U Joint Snap Rings

[Wildlife]

My 2005 SE 4X4 has a very slight vibration that feels like it might be drive line related. It has an aluminum driveshaft with only 2 U joints. The rear joint had slight (less than .002") side-to-side movement, so I ordered a replacement. In reading the Nissan Service Manual, section PR (prop shaft), on the assembly instructions it says to select the correct snap ring to adjust the play to less than .0008" (8 ten thousandths - less than .001). On the last page of the document, it provides 4 part numbers of different thickness snap rings to adjust the play. These snap rings are all obsolete (unavailable).

Does anyone know why they were discontinued?
Is there an alternate method to adjust U joint play?
Is it just unnecessary to be concerned about the play?