The Crazy Town boys are living up to their band name -- 2 of the members got into a good old-fashioned fistfight after a show this weekend ... and it was ugly.

TMZ has obtained video of the throwdown that unfolded Sunday in Myrtle Beach, SC -- where CT was performing ... only to have their lead singer, Shifty Shellshock, not show up 'til the very end. Co-vocalist Bobby Reeves had to fill in ... making for a very rocky show.

There's video circulating of Reeves attempting to handle frontman duties -- at times, forgetting lyrics to songs -- while publicly expressing his frustration with Shifty for bailing.

Eventually, Shifty did show up and another clip that's made the rounds shows him and Reeves in a heated argument by an SUV -- with BR rocking a fresh black eye. Now, we know how he got that shiner ... they were trading haymakers after the Myrtle Beach gig.

In our video, you can see Shifty wailing on Bobby ... and they're arguing about money SS believes Bobby stole. Bobby denies that, and goes on to make some pretty vile threats against Shifty's family members ... including his kids.

Eventually, Bobby sneaks in a sucker punch and they go at it again -- only to finally be pulled apart by others. Sources close to the band tell TMZ this brouhaha went down as soon as they got off stage, but in the aftermath, they've supposedly settled things.

We're told it was a "fight between brothers." Unfortunately, this is just the latest mishap in Crazy Town -- one night prior, Shifty got into it with a sound guy during a show in North Carolina. FWIW, the guy's publicly battled addiction for years.

We'll see how long the band peace treaty lasts.