This is low comedy of a high order, the rowdiest western jokefest since Blazing Saddles. Billy Crystal stars as Mitch Robbins, a Manhattan radio-ad salesman with a wife (Patricia Wettig), two kids and a ten-ton midlife crisis brought on by his fortieth birthday. To cheer up Mitch, his two best buddies -- extroverted sporting-goods exec Ed Furillo (Bruno Kirby) and timid grocer Phil Berquist (Daniel Stern) -- take him on vacation to a working dude ranch. Mitch's pals have troubles, too. Ed, who…