Man, that stinks. Were those all full? Hopefully most survived.
balatonwine's father has some solid advice. I've never loaded a pallet jack much higher than the handle and that was always with sturdy construction material. If you need to stack that high for whatever unavoidable reason (limited space, etc.), then I would suggest wrapping with the stuff that looks like saran wrap that they use for shipping and putting some sort of support (heavy cardboard) every few rows.