I was rear-ended by a kid driving a chevy s-10 blazer at about 45 mph while I was at a stop light. I suffered some whiplash, my daughter, who was 17 months old at the time, was uninjured. The reason? We were sitting in a big old Ford F-250 with a solid frame. The kid in the blazer was taken away on a stretcher.
Had I been driving a Geo Metro or something "environmentally approved" or with "crumple zones" we would have been propelled into cross traffic (also traveling 45 mph) and at a minimum, seriously injured.
I don't know what kind of "truck" you have that wouldn't come out of a 5 mph "collision" unscathed, but, I suggest you invest a little money in a real bumper. I back into stumps and hybrid cars all the time and there's no problems.