The hip-hop juggernaut once known as Marshall Mathers could be forgiven for seeking the kind of cocktails-and-cabanas vacation rappers are fond of re-creating in lavish video clips.

In the two years since his last full-length album, “Encore,” Eminem has mourned the shooting death of a friend he frequently described as a brother, the rapper Deshaun “Proof” Holton; remarried and re-divorced estranged wife Kim Mathers in the span of three months; suffered a betrayal at the hands of a close associate, the highly sought-after DJ Green Lantern; and bowed out of his Anger Management world tour to seek rehabilitation for an addiction to sleep medication.

While he recovered from exhaustion, an album of his greatest hits, “Curtain Call,” arrived in stores, prompting rumors of impending retirement. Late last year, Eminem dismissed the gossip but conceded he needed a break. The release today of “Eminem Presents: The Re-Up”(Shady Records),a mixtape that likely precedes his fifth studio album, proves there is just no rest for the weary, Detroit-born rapper.

In hip-hop parlance, to “re-up” (a riff on the military term) is to make battle, to put adversaries on notice in no uncertain terms. But a tumultuous couple of years notwithstanding, it may be difficult to figure why the best-selling rapper of all time needed a call to arms.

Apparently, Eminem had his ear pressed close to “the streets” during his hiatus. On the opening track, “We’re Back,” he confronts rap’s kingmakers: “All I hear is I’m the best at this and best at that / But I don’t hear my name no never brought up in rap.” After lamenting his lack of recognition in the urban court of public opinion, Eminem counters growing fan backlash that his music has gone pop, played “on the radio 20 times in a row daily.” “I thought the formula was to hit mainstream and make it big, buh-big, big,” he growls over a stunningly sinister bass line.

With assistance from producer the Alchemist, “The Re-Up” gathers an army of Eminem’s comrades and Shady Records label mates: 50 Cent, rap group D-12 (which contributes posthumous appearances by slain member Proof), lyrical wonder boy Obie Trice, and new soldiers, including Stat Quo and Ca$his. Gone are the squeaky, frat-rap lyrics of songs such as “My Band.”Instead, fueled here by almost palpable bile, an introspective Eminem unleashes verse after verse of clever wordplay.

To understand Eminem’s use of the mixtape to vent frustration is to grasp the urgency of the mixtape, a medium by which rap fortunes can be made or annulled. In the mid to late 1980s, as rap grew in currency, fledgling New York city DJs evolved the practice of hosting their own cassette tape recordings, blending or remixing hit rap songs over new beats, singing freestyle verses over familiar melodic lines, and almost always including a healthy dose of selfaggrandizing voice-over.

Rappers soon caught on to an invaluable method of reaching an inner-city audience; the scarcity of the homegrown tapes lent them instant exclusivity. These days, leading DJs such as Kay Slay, Whoo Kid (a presence on this disc), and Big Mike enjoy direct access to record company vaults and are often charged with manufacturing advance buzz for both newly signed and established artists. It has become de rigueur for hip-hop artists to make appearances on or employ DJs to helm mixtapes — now CDs — where beefs are settled, anticipation is built for an album, or, as with Eminem, the case is made that success hasn’t washed from him the grime that lines Eight Mile.

The inexhaustible desire to stamp the brand of authenticity on new releases is what bridges the gap between an underground mixtape artist and a mainstream rapper like Eminem. Hiphop is still a business that thrives on unsigned hype, and Eminem is well-versed in the power of the mixtape as a tool to bestow cachet. In the 1990s, then aspiring rapper 50 Cent collaborated with fellow Queens native Sha Money XL on a barrage of mixtapes. The pair flooded the market with a product that eventually made its way from street-level sellers into the hands of Eminem, who later entered a bidding war for 50 Cent and eventually signed the rapper to an unprecedented seven-figure joint venture deal with Shady Records and Dr. Dre’s Aftermath label in 2001.

“The Re-Up” mixtape comes packaged as a makeover of sorts for a rapper who believes he has been inadvertently tarnished by the stain of pop stardom. If the album is weighed down by the more mediocre members of Eminem’s roster, it should serve at least to whet the appetite for the solo album to come.