Ndlovu's current contract expires at the end of June and Blades boss Neil Warnock has offered him a two-year extension.
Veteran midfielder McCall and defender Kozluk have been offered one-year deals.
Morrison, who joined the club on a short-term deal from city rivals Sheffield Wednesday, is to hold talks with Warnock over an extension.
Meanwhile, long-serving goalkeeper Simon Tracey and versatile performer Robert Ullathorne have been released by the club.
Tracey missed the whole of last season through injury, while Ullathorne has been missing since October.
Youngsters Ben Doane, Gary Kelly, Lewis Killeen, Ryan Mallon, Grant Smith and Tyrone Thompson have all been told they can leave as they do not figure in Warnock's plans.
