What has most likely happened is the extruded channel on the inside of the aluminum bumper has corroded away. The shocks are secured in place with special bolts, these bolts have square flat heads which fit into the aluminum channel. The bumper shocks fit over the special bolts and washers and nuts secure the shocks in place. When the aluminum corrodes away from the edges of the extruded channel there is nothing to hold the special bolt's heads.

Replacing the bumper is one option, the other option is to remove the rubber impact strip and inside the channel the impact strip fits into, drill a pair of holes thru the aluminum bumper and through the shock’s mounting plates, bolt the shock to the bumper, and then you reinstall the rubber impact strip to cover the bolt heads. Because the bolt heads conflict with the back side of the rubber impact strip you can do a couple things either separately or in combination. Countersink the bolt head, this works best for slotted flat head type bolts, grind down the bolt head after installation to reduce its thickness, or carefully grind/melt out depressions in the back side of the rubber impact strips. With this done the impact strip will not show bumps where the bolt heads are.

When it comes time to reinstall the impact strip the warmer it is the better it will go! Do this with the bumper off with its channel facing upwards and where you can apply downward pressure while simultaneously moving the strip left and right working one side into the channel then the other, you’ll usually need a rubber mallet for the ends, warmth really helps when you get to the end sections