Left-wing ideology shouldn't be reduced to mere 'utilitarian egalitarianism'. As I previously argued paradoxes with intuitive rights can be overcome by simply granting those rights status of untouchability but I'd like to add one important aspect here:

Your moral examples aren't really equal and thus comparable for at least three reasons:

(1)Reason of poverty and excess wealth in aggregate can be accounted for systemic unfairnesses in the economy. Interest is one mechanism that greatly favors those already wealthy. Reason for kidney failure, on the other hand, is mostly sheer bad luck.
(2)Transfer of wealth can be done equally among certain group, organ transfers can't. Thus forced kidney 'donation' is against the principle of equality as all equal individuals don't have to donate a kidney, but one would have to go with random donors.
(3)Forced transfer of wealth has support of the majority, forced transfer of kidneys don't.

As a western European I've never heard the argument that you wouldn't have legitimacy over property you don't use. Not really even by the insignificantly small (0.3%-0.5%) communist minority (and by communist I mean real communists, not the 'has-a-flavor-of-red-communist' you see thrown around in these circles. They, as you might imagine, have their own ideas of property). Thus I consider your original argument be bit of a straw man so I took the right to speak for transfer of wealth in general.

So, in conclusion we can say that transfer of wealth has legitimacy because (i) the systemic problems that lead to poverty can be attributed to every individual ('caused by the collective'), (ii) according correction can be done collectively ('fairly') and (iii) majority thinks its reasonable policy. Forced kidney donations don't have legitimacy because (i) kidney failure is unrelated to other people, (ii) the forced donation can't be done according to principles of equality and (iii) it is against majority's opinion of what is right.

Left-wing ideologies tend to emphasize democracy and so I think from this perspective its unfounded to use anything as counter-example that wouldn't pass for majority opinions. It is of course valid point to ask if something is right because majority thinks so, but currently it grants legitimacy.

Edit: To summarize, I'd would say OP made a straw man on that left-wingers are all about marginal utility.