Jessica Lange, with Hannah Shepard and Samuel Walker Shepard – the children, now grown, that she had with Sam Shepard – entered the Ellen Stewart Theatre quietly as an audience of age-burnished talent from the electrifying free-form heyday of off-off Broadway theater waved greetings, blew kisses, shared memories and settled in Sunday for an afternoon tribute to the actor and playwright, who died July 27 at 73. The master of ceremonies was Jean-Claude van Itallie, whose…