ring and pinion have nothing to do with the axle spline count. Any normal rotation 8.8 ring and pinion will work.
as far as axle spline count, the carrier, either an open, a trac-loc, or some other type of locker, that is when you have you worry about spline count, and you need 28.
the side gears in the carrier for IRS rears have a bevel since the axles have a snap ring. If the carrier you use doesn't have that bevel then it can be nearly impossible to remove the axles again if the snap ring is left in place. Some just leave it off, but that doesn't sound like a good idea to me.
If you are also getting a new carrier that is something you need to take into consideration. I think all current trac-locs sold by FRPP have the correct side gears (the IRS ones work for solid axles also).
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