Here the typesetter for the 1852 LDS edition accidentally set the noun strength instead of soul in the phrase “and your whole soul”, undoubtedly because of the immediately preceding occurrence of strength. For the second printing of the edition, the appropriate soul was restored (by correcting the stereotyped plates).
Elsewhere the text has no examples of “whole strength”, but there are six examples of “whole soul(s)”, which read as follows in the earliest text:
Summary: Maintain in 2 Nephi 25:29 the original reading “with all your might mind and strength and your whole soul”.