As far as I know, the only way to get your backpack from unreachable areas is to use the Recover Backpack option under the Game Repair. You can only do it once per save load so if it happens again you'll have to reload a save.
My cousin and I have also had this problem. The work around we found was to build an Ant Nest, store the buggy that's "following" us, then withdraw it. It spawns at the nest and will return to it if it gets knocked out again in the future.
