I replaced my rear bumper (center section) a few weeks ago. I was not that bad, it looks great now. However, I'll share are few of my troubles so you can watch out for them.
I had to buy three bumpers before I found one that had all the mounting tabs welded correctly. I knew I was buying a $95 bumper so it was not a surprise but still, be prepared to encounter this unless you buy a Toyota bumper. I think Toyota bumpers are much better but I ended up getting the aftermarket one to work very well. Just took more time.
I used all new bolts and recommend them. They cost $30 from Toyota, sorry I don't have part #s. There were about 16 bolts IIRC. You may be able to find the right ones at a hardware store. Point is, new bolts make reinstall much nicer. I also used anti seize when I reinstalled the bumper in case i need to take it off for any reason later.
Most importantly, after trying PB Blaster, I found that Kroil Areo Oil is the best! I bought mine from Amazon.com. It was much better than PB Blaster. I also had to use some heat/propane torch. Spray on rusty bolt threads and let it sit overnight if you can.
One more thing...I had trouble getting the part#12 above reinstalled. The cheap bumper did not have the holes in the correct location; just on one of the holes for this piece was not in the exact right location. I chose to enlarge the existing hole and "making it work".
And another thing...I used a floor jack to help with final alignment. Do not tighten all the bolts as you assemble. Just get them started because you will need to massage the bumper and the sides in to alignment. When it is all straight, you can tighten the bolts. Be careful not to strip the bolts welded on the cheap bumper. If a bolt is not threading correctly stop and try again, don't force them. I also used a ratchet strap to help with final alignment. Use what ever you have. Be careful with a hammer, you will probably need a rubber mallet but if you swing too hard, you can brake the mounting bracket welds. Ask me how I know.
Some bolts are more important to not brake than others. Good luck, it is not hard but the rusty bolts do not help. Other than the rusty bolts it is super easy. Yes, you need to remove the spare tire, mud flaps, the metal bumper end caps and the plastic step piece.
