Grade 5 bolts are more than strong enough to use for the transfer case skidplate, Grade 8 are even stronger but you don't need Grade 8 there. Grade 5 and 8 bolts are simply much stronger than your average garden-variety hardware.
Sometimes you'll get lucky and find such bolts at Lowes but stores like Lowes, Home Depot, etc. rarely have much of a size selection of hardened bolts and nuts to choose from. More often you will need to go to a smaller specialty hardware store to find them. You simply need 1" longer versions of what you have now.
By the way, Grade 5 and Grade 8 refers to hardware that is in an SAE (U.S.) size like 1/2". Newer Wranglers use Metric size bolts there which do not come in Grade 5 or Grade 8, you will want to use a Metric 8.8 bolt there which is roughly equal to a Grade 5. Metric 10.9 is roughly a Grade 8.