If you're coming from Brisbane, you could turn at Blacksoil towards Fernvale and Esk and turn onto the Esk-Hampton Road and then head to Toowoomba from Hampton, it's about 50km longer than going up the range, but with all the road works it may be quicker.
If you're as far north as Burpengary or Caboolture, the quickest way is Woodford, Kilcoy, Toogoolawah, Crows Nest to Toowoomba
Another way to avoid the range itself is to take the Spring Bluff/Murphys Creek turnoff just after you get off the Gatton bypass. Its nearly 20km longer, but depends how long it takes to get up the range.