Here are a few things I did to a beater jeep to lighten it up a bit. Throw the tailgate away and ratchet strap the spare tire to the roll bar. Sell the hood, top and doors. Remove some of the bolts that are overkill. Fill the tires with helium. Use a hole saw and drill the frame in a few spots and run around in the salt in the winter time and it will get alot lighter. With fewer bolts the repairs were much quicker and the chance of breaking off a rusty bolt was lessend.
After all of this the fuel mileage and off road capability remained the same. I found that the frame had decided to bend permanently and 3 wheel motion (hydraulic low rider terms) was common while turning so I jumped it across a creek for its last big hoorah!
The tailgate idea sounds good, but to make it weather proof and lighter will be a challenge!