Scoremaster,
I believe the primary differences in shellcracker and crappie jigs are the size and color.
Crappie jigs are generally more colorful because of their bites are often reactionary and defensive - not always to feed, particularly during the spawn. Bright colors like chart, flo pink, and flo green work well because they are attention grabbers and elicit a feeding, but also aggressive response. Crappies also feed/bite things that are above them because of the position of their eyes. That's why you always hear of people misjudging the depth of crappie and fishing "under" them. They just can't see the bait. Therefore, feathers and other materials that provide good action are very important with crappies. Crackers on the other hand are bottom feeders. They go after small crustaceans, worms, and other things living on the bottom. They are generall smaller than crappie, but bigger than bluegills. Therefore, the ideal jig for crackers would be smaller than for a crappie, prolly something in the 1/32, 1/48, or 1/64 size range. Also the colors used for cracker jigs should more closely resemble their primary food source such as black, green, and dark oranges as opposed to trying to get reactionary bites with bright colors. Last, brim will bit just about anything you put in front of them that they can get their mouth around. I have caught them on dark and colorful jigs, buy they tend to favor smaller, darker jigs with legs and/or wings. They also respond extremely well to scents.
I am still new to tying though, so keep that in mind when taking my advice on this on the part about how to tie the jigs
. The part about general fishing is reliable though :D.
Mistah Kurtz - he dead
A penny for the Old Guy