how do the Desert Rangers have LAPD SWAT armor before they join the NCR if there based in Nevada and Arizona?

That style of armor was used by the marines, minus the trench coat, like this. http://fallout.wikia.com/wiki/Desert_Ranger_combat_armor

Now chances are the desert rangers used this style of equipment, and there weren't too many of them. The NCR then comes and signs the treaty and absorbs them into the ranger program. The NCR military is pretty big and eventually more and more rangers advance to the Veteran Status along with the Desert Rangers. Instead of looking for more marine armor they could have simply used old LAPD riot gear.

who says its LAPD SWAT?

The armor has a big LAPD on it.

Really? I never noticed LAPD on Desert Ranger Combat armor, I even looked. where is it located?

No the NCR Veteran Ranger armor.

There armor is based off of desert ranger armor which is what I was trying to explain.

Yeah, I know:)

the OP says the Desert Rangers have LAPD armor. I asked "who says its LAPD SWAT," because if it doesn't say LAPD then its just modified combat armor.

well the Ranger vets are manly made up of Desert Rangers with older NCR ranger joining them right?