Being a heavy duty just means it is more capable, it doesn't give it any additional tow rating, or load carrying capacity that I am aware of. (of course, I may be missing something to0.....)
Its possible the original t-case crapped out, and that was what they put in there. The tag should read 231DHD....... If you can find one of the HD units, it should be a direct bolt in.
Of course, the 241 would bolt right up as well, to the trans anyway.... but, rear output shaft is different spline count/diameter... I think, and the front shaft uses larger u-joints.... not sure if the flange is larger or not.