can you make random items drop from random blocks?

You can set up a dropper that shoots an item/block when a /testfor @e[type=item,name="Item Name",c=1] returns true.

Then have a command block that waits say 5 ticks to fire that teleports the dropped item (/tp @e[type=item,r=3,name="Alternate Item"] @e[type=item,name="Item Name",c=1]) and after that /kill @e[type=item,name="Item Name",c=1] the first item.

You can use this method for randomization once you've got the basics set up.