HELP! When Braking

for some reason when I brake hard there is a loud crunching noise from the rear passenger side which also causes the van ( 2001 venture wbe ) to shift. I went about a month ago to change my front brakes , and also had the guy look at the rear, he said they where fine and would last much longer. When I brake softley there is no noise no jerking no problem. Does anyone have any idea what might be the problem before I have to take it in to the shop?

Any help , suggestions , is more than I have now.