So, OK. Diffs are expensive. They average around 5-6 hours labor to pull one and reseal it. At my shop we are 190$ an hour .... So yeah. Can't speak on the diff you are looking at but the mount plate could be the way to go.
Fuel tank vent, or activated charcoal canister shut off valve, this thing will be next to your charcoal canister. Should be in the rear near the fuel tank. Follow the lines from the tank. I think yours in under the car but sometimes they sit in the wheel wells.
Google it, try charcoal shit off valve or something or just search for the canister.
Yes your seat belt tensioner is related to your airbag light. Honestly I would unplug it and plug it back in a couple of times (with the key off) then just have the codes cleared and see if it comes back.
Your parking brake may just need adjustment at the rear wheels. I bet the rear disc has an internal drum brake for the parking brake.... If so they need to be adjusted. Or if it's a caliper with a cable you may just need to adjust the cable tension. Both are cable systems but the adjustments vary.
Sounds like you have a project
