Possible hidden damage when rear ended?

2002 Ford Ranger extended cab 67000mi. Manual trans. I was rear ended. I was stopped, he hit me at maybe 25 mph? Twice! Passengerside. Appeared to have only messed up the bumper, however, last night heard a loud "sproing" sound and saw dents poking back out of the bumper like something under pressure let loose. Can see evidence (clips and bars) under the truck that the bed or frame must have moved forward on the passenger side 1/2" or less. Insurance has only looked at cosmetic repairs and I want to be sure I don't get stuck with something more down the road. Any suggestions as to what else to have someone look for? Thanks!