What is the best way of going about taking off the TOYOTA decals on the tailgate...?

If you have a drill you can go down to the autoparts store and get a "eraser" type thing for like 11 bucks. You just follow the directions and use it to take the decals off. You will have some smudging at first, I recommend not using to much pressure.

What is the best way of going about taking off the TOYOTA decals on the tailgate...?

I used a hair dryer to heat them up a bit and then just peeled them off. You will have to do a bit of buffing by hand to shine the rest of the tailgate to match the shinny paint from where the stickers were.