I'll do my best to help you out, as I just went through this with my '04 Armada LE recently. In my case, my spoiler flew off on the highway, took the light and washer nozzle with it, and got smashed into tiny little pieces by a big rig. You shouldn't worry though, I believe Nissan re-designed the attachment system with the '05 spoiler, which is what I used for a replacement.
Anyway....the black bar to which you're referring is the spoiler itself. And as far as I could tell, there's no way to replace the light without removing the spoiler. You are most of the way there with the removal of the nuts. I thought there may be 5 total, can't recall. Might just be 3. The bad news is that the spoiler is also held down with a series of single-use plastic pop-in type clips, and a strip of adhesive, double-sided tape at the front of the spoiler. (You may or may not have adhesive, but definitely should have clips). So it will take some force to pop the spoiler off, but be careful, because being too aggressive my break the clip mounting spots within the spoiler, and not pop the clips themselves. I suppose you could use a utility knife to slice the adhesive (about 1" wide, I think), and then use a long slotted screwdriver to try to pop the clips. Not sure, as I took mine off the easy way (wind).
Once you do remove the spoiler, you most likely won't be able to put it back on without new clips and adhesive. You might be able to get them at the dealer, but I ordered everything I needed through Courtesy Parts Nissan, I think. It was like $20 or so for the adhesive kit, and just a few bucks for new clips and nuts/bolts. Either Courtesy Parts or mynissanparts.com should have everything you need. The re-installation is easy to do (comes with good instructions), but you just need the hardware first. No biggie.
One caveat: the 2 mounting nuts that I ordered with my brake light wouldn't work with the mounting posts on the spoiler, so I had to get a little creative with how I attached the light (from the inside of the spoiler, of course) to the mounting posts. I think I used different locknuts. In your case, you are just removing the old light, and swapping in the new one. Everything else should be the original equipment, so it should be a non-issue.
Hope this helps. Feel free to PM me if you need more clarification.