We have a slab floored Florida home. After 5 years, some of the tiles sound hollow, as if there's a space under them. Is it possible to just remove some grout and force some sort of sealer under them. I'm afraid they might crack if stepped on just wrong.

I don't know of any product you could inject under the tile that would properly remedy your situation seeing that the problem may lay out of reach. Obviously you have adhesion failure due to any number of improper install issues or an underlying crack formed under the tiles in question.
The proper remedy (and this is where a supply of extra floor tile stored somewhere in the house will come in handy in case of breakage) is to carefully remove the grout from around the loose tile/s and attempt to carefully remove the loose tile in one whole piece then scrape the old mortar from the floor and as much as you can from the back of the floor tile and thoroughly clean and reset the tile/s using properly mixed fresh mortar and then regrout. If you find a crack in the slab that will open another can of worms! How many tiles are in question and does this hollow sound possibly run in a line?