Solution that DOES NOT
require a grenade jump...
The location for this skull is the level Assault On The Control Room. It is in the room you enter when you take the first elevator down, and immediately before you head into the open area with the first wraith. The usual solution requires a well-timed grenade jump, which is something I (and many others) suck at, quite frankly.
Having spent far too long failing to do the grenade jump correctly, I decided to continue the level and try again later, and headed out into the area outside. Turning left I noted the marines pinned down with an upside-down warthog, and wondered if it would be possible to get the 'hog back into the room with the skull. Turns out that it is.
The doors leading back inside won't open if you're in the warthog, so drive up to them, get out and wait for the doors to open before getting back in. The corridors are surprisingly easy to navigate in the 'hog, and you should even be able to get around the pillars in the room with little effort. Helpfully, the gun on the back even manages to move through
the scenery at one point...
Drive up the right-hand edge of the platform you would have grenade-jumped from (at the back of the open space in the middle of the room) and along until you're at the centre. Face the vehicle upward, jump out and it should slide down into position. You can now jump on the warthog and make it to the pillar and the skull without too much difficulty.
Video added showing where to position the 'hog, and where to find the skull when you've done so.
Edit: EDIOS BUDVAR
has a great extra tip. Running in two-player, get player 2 on the warthog and jump on top of them to get yourself higher and make the jump a lot easier.