Jotun Grunt and upkeep question

[kombat]

How does the timing work with Jotun Grunt, with respect to choosing the graveyard cards to remove from the game?  Given that Jotun Grunt doesn't say "target," can I wait to see which cards my opponent wishes to remove, then respond (say, by welding it into play)?  For example, if I have a Crucible and a Tormod's Crypt in my graveyard (in addition to several more cards), and during my opponent's upkeep, he's paying the cumulative upkeep for his Jotun Grunt and announces he wishes to remove Tormod's Crypt and Crucible from my graveyard.  Can I then weld the Crucible into play, instead of removing it?  I suspect the answer is "no."

If the answer is in fact "yes," then what happens to the Jotun Grunt? He was required to remove 2 cards from the game, but he may have only gotten 1.  Does that mean he dies?  Does his controller select another card instead?  Does he stay in play anyway, since he "tried" to remove 2 cards?

[Dante]

please post card's game text when asking questions regarding a card or the rules text if asking about a rule.  Verbal warning