Madlib and Dilla worked together on a few occasions, and developed a close bond. Their most notable work together was Champion Sound, released by their duo JayLib. It features them each rapping over beats that the other has produced. Here is J Dilla speaking on his relationship with Madlib.
http://www.youtube.com/watch?v=uRzkGVW7z0Q

Julio G was a west coast DJ out of LA who worked on KDAY 93.5 radio station. He has worked with many of the large west coast hip hop artist since leaving KDAY.

Side Note- You may also realize him from Radio Los Santos in GTA San Adnreas

Public Enemy is a highly influential hip hop group from Long Island, New York. They are known as being heavily influential in the political hip hop scene, and are recognized as legends. Their time of greatest success came in the late 80s and early 90s with the classic albums It Takes a Nation of Millions to Hold Us Back and Fear of a Black Planet.

From left to right: DJ Terminator X, Hype man Flava Flav, Emcee Chuck D

Soulsonic Force was a 1980s hip hop group with producer DJ Afrika Bambaataa that pioneered much of modern dance music. Bambaataa was a legend in his own right, being an originator of break beat deejaying and for development of the Universal Zulu Nation. The groups most well known and influential song is Planet Rock.

Quas mentions two early hip hop groups in this line. The first of which is Ultramagnetic MCs. This group came out of the Bronx in the mid 80s, with members Kool Keith, Ced Gee, TR Love, and Moe Love.

The second group, Stetsasonic, came out of Brooklyn in the late 70s. The most famous and successful member being Prince Paul.

Here the subjects are driving in their Cadillac, which might refer to what seem to be a pink Cadillac featured on the cover of the album

Rolling joints from their sack of weed

Freebasing refers to using a drug in its non-soluble form. This usually means that it cannot be ingested or snorted, but must be smoked. The most common freebase drugs are crack cocaine or heroin. The last part of the line refers to chasing the ghost, which is the act of collecting one last hit of the drug. After most of it has been consumed, the user will collect the residue left, and finish off the day. The following audio clip suggest that in this instance Quas is referring to heroin.

Freebase cocaine/crack cocaine

Quas referring to his more well known identity here Madlib. Madlib is originally from Oxnard California, where he started his career before moving to Los Angeles (hub of the west coast rap scene). This line also references manifest destiny, the idea that Americans were destined to expand westward across the continent.

A painting by John Gast, representing the movement of civilization westward.