My Jeep's rack on the roof (stock) was faded to almost white when I bought it. There's still the original black remaining mainly on the sides. Think if I spraypainted it all black that it would survive the heat of the sun and not peel off?

 

A lot of people use bed liner on the side trim which fades the same way as your describing and I've heard good results. Bed liner would hold up much better than regular spray paint.

The key to a good paint job is in the prep. 90 percent prep 10 percent paint.

 

yeah my roof rack rails on my 01 sequoia where faded bad so i painted them up and it held up fine, but its cracking and peeling really bad now plus i got over spray on the top of the roof that wont come off.

just prep them with some sandpaper, use a few coats of bedliner and you should be good to go.
(i would wax your whole jeep first just incase you get overspray so it wont stick to the paint. overspray sucks)