I installed one of these on my 04 Titan (you won't need it on 05+ since they have the tailgate assist from the factory). Best $20.00 plus S/H I spent on the Titan so far. It's basically a torsion bar that reduces to effort needed to lower and close the tailgate.
It's a very simple product, but performs quite well. I can bring down the tailgate slowly and close it with only one hand without much effort. The install is fairly easy, requiring you to drill two holes, one in the sheetmetal near the left taillight and the other underneath the tailgate. Thehardest part is actually putting the tailgate onto the torsion bar as you have to hold the tailgate at an angle to get it in correctly. It will require at least 2 people, but 3 would make it much simpler. The only real downside is that you will have to drill into sheet metal, but it is not easily visible and compared to the cost of trying to modify an 04 to accept the 05+ hardware, it's well worth it.