After Post impressed with a solo performance of his songs, "I Had Some Help" and "Never Love You Again," Reba joined PM onstage at the Ford Center in Frisco, TX ... asking him to indulge her with a surprise collab.

Specifically, Reba invited Posty to join her for a performance of the Allman Brothers Band's 1973 hit, "Ramblin' Man" ... in honor of the late Dickey Betts.

Remember, Dickey -- the lead guitarist for the Allman Bros -- died on April 18 at home in Osprey, Florida following a battle with cancer. He was 80 years old.

Post was, unsurprisingly, game for the duet ... offering Reba a G-chord before she launched into their impromptu, a capella rendition.

Reba and Post took turns with the song's lyrics, before harmonizing with one another during the chorus -- leaving the crowd thoroughly impressed.

It's moments like these that have secured Reba's longtime hosting gig of the ACM Awards ... and the awards show already confirmed she'll be back again next year.

Of course, Reba and Post weren't the only notable names to take the stage ... as Lainey Wilson, Jelly Roll, Miranda Lambert, Thomas Rhett, Blake Shelton and more country favorites performed.