Stir Jell-O into the 2 cups boiling water until completely dissolved. Combine reserved pineapple juice with enough cold water to make 1 1/2 cups. Stir the cold liquid into the Jell-O. Refrigerate this for 1 1/2 to 2 hours, until thickened enough to hold fruit in suspension.
When beginning to set, stir in the drained pineapple, cabbage, and carrots until well-mixed. Pour into a 2 quart casserole dish or gelatin mold, or an 8x8x2-inch pan. Refrigerate until firm.
This may be unmolded onto a serving plate if you desire, but I always just leave it in the dish and scoop it out to serve.
Sometimes I mix 1/2 cup Miracle Whip with about 2/3 cup cool whip until well blended, and spread it over the top, or else dollop on top of the servings, but this is not necessary.