I make most all of my own recovery gear, including soft shackles. For a 6000lb vehicle you should be looking for a unit constructed from a 10mm or 7/16" Dyneema, something SK75 or Amsteel Blue. A properly done unit in that size material should have a MBS (Minimum Breaking Strength) of more than 40klbs. I look at most 'drag' recovery ( not overhead lifting ) with a FOS ( Factor Of Safety ) of 2-4:1 depending on what type of device it is. A soft shackle is going to see the most wear in the system typically, so 4:1 is more applicable. On a synthetic winch line, 2:1 is going to be about all you can reasonably do.
A quality soft shackle is going to be $30-40+ typically.