It seems there is no ability to build armoured and tracked rocket artillery pieces. In history the primary German self-propelled rocket launchers were based on halftrack designs, some even on converted M3 Halftracks (Wikipedia). As a person who has himself taken part in a wargame as part of a mechanised force in a self-propelled artillery unit, I know that artillery can meet enemy infantry. The light armour on such vehicles allows them to be used much more safely in rapid advances without having to have frontline units babysit them all the time, this should lead to an increase in organisation. The tracks on these vehicles allows them better cross terrain ability than wheeled vehicles. I think either the first or second mechanised infantry tech should have a rocket arty variant.