Rebels Phnom SAIGON (UPl)—South Vietnamese forces seeking a morale-boosting victory drove back a Communist assault on the surrounded key city of Xuan Loc yesterday and also reopened Saigon’s vital road to the Mekong Delta rice bowl. Despite these two successes, however, the battle for Xuan Loc raged on with the North Vietnamese using the steeple of a downtown Roman Catholic church as their aiming point to pour artillery shells into the ruined city. The Saigon command reported ,22 Communist soldiers killed in ,/the abortive attack outside Xuan Loc at Thm Giao village. Spokesmen said there were no government casualties. While Xuan Loc, 38 miles northeast of Saigon, holds out, the Communists are unable to mount their full force for an assault on Saigon itself and so the battle for the city is considered perhaps the most important in the, 30-year Indochina struggle. 1 South Vietnamese officers say a victory at Xuan Loc is necessary to prove government troops can hold their lines. Massive retreats last month sent the morale of soldiers and citizens plummeting. Spokesman said 153 artillery shells hit S. Viets road to PHNOM