The rear end will sometimes bottom out when going over a bump.
What makes my 4Runner ride so rough on the rear end. on 1999 Toyota 4Runner
Stock suspension and tires? check springs and shocks.
I'm late to the party on this one but I will throw in my 2-cents for others who might have this same issue. More than likely one of your bushings on the top of your rear shock OR a bad shock. I just had this issue about 4 months ago where just pulling out my driveway onto the street with that little 1 1/2" drop would make the back of my 4Runner sound like it was falling apart. The bushing was gone. replaced it for $3 and i was good to go.