It's impossible to drive a Pelican through a glitch. It is likely that in Halo 2, it was simply a piece of BSP geometry, as the Scarab was. In Halo: Combat Evolved, the Pelican was an actual vehicle. But it is only drivable through modding; there are no glitches.
Look up "how do i drive a pelican on reach?" on here. I wrote a clearly-stepped guide on how to drive a Pelican on Reach. It's legit and it should be easy to follow. AND IT'S AN EASTER EGG LUBASDSFASDSDASDASD!!!!