There was a major exhaust leak, which we just assumed was at the manifolds since this vehicle obviously towed a heavy camper most of its life. But while checking the vacuum lines to the T-case at the firewall we felt a blast of exhaust gassed from the EGR valve. It had been installed with no gasket. To avoid a parts store run, we simply slathered it with Permatex Ultra Copper RTV and slapped it back on. Leak fixed, hack accomplished.