Garage shelf supports splitting and need to be supported. What is best?
You could try "sistering" another piece of lumber to the back (inside) of the horizontal supports.  Apply glue and put a few screws in from the front, and it should lend some additional strength to the edges of the shelves.  You might even be able to jack up the shelf a bit in-place before sistering to lend additional resistance.  (Just be careful.  Jacks can tip, and then your project could literally come crashing down around you)

EDIT: Just don't put in a bajillion scews, or it could aggravate your problem.  Looking at your shelf some more, you could also consider jacking up the shelf and putting another layer of plywood on the bottom.  This would turn your shelf into more of a box, and would lend some additional rigidity.