Grade 8 is some serious overkill, since there's really no stress on the bolt. I suspect the originals are grade 2 or so.

 

Just changed my front & rear differential fluids with royal purple 75-140 for the rear, 75-90 for the front. Also did the transfer case. Also changed the differential cover, never really liked the fact that the stock rear diff. cover had little more that a rubber plug plugging the fill hole. Mine was seeping a little from this plug. Wire brushed the rear axle and gave 'er a fresh coat of tremclad (Rustoleum). The chrome rear diff. cover came with new stainless grade 5 bolts. Here are some pics. Just have to touch up under the new diff. The new chrome diff has a screw in fill hole plug.

Before:

After:

 

I know that Lowes carries 5/16-18 x 3/4 bolts.Not sure how to rate that against hardened steel,but stainless is very tough/hard and will not (under most circumstances) rust either.

 

Stock are grade 5, per the parts # catalog, but after thinking about it for a while, grade 2 should be sufficient due to the yield strength and tensile strength. If this were a Jeep or something that I was going to be doing serious offroading with, then I could make the case for going with a high grade bolt that won't shear as easily. But since I won't be doing any rock crawling with my truck, the grade 2's should be fine. Thanks for all the replies!