Does the husband's saying to his wife, "Hold your uncle," meaning his penis, constitute ẓihār, especially given that he said it jokingly and laughingly, and he had previously said that to her and she had held it?

Zihar is when a husband compares his wife, or a part of her, to a woman with whom he is forbidden to marry (a mahram (definition)), such as his mother. The pronouncements of zihar can be explicit, like "You are to me as the back of my mother," and this takes effect without the intention. They can also be allusive, like "You are to me as my mother," and this takes effect with the intention. If he did not intend zihar with such a phrase, then there is no consequence. The wording mentioned in the question is not considered zihar because it does not involve comparing the wife to a forbidden woman.