How do I fix the vibration issue on my 2012 Holden Colorado?

Your problem could well be torque converter-related, but it could also be something else inside the transmission. The fact that the transmission will not shift back out of sixth gear even going uphill, suggests that there’s – in theory least – a problem with the electronics or hydraulics that control the shifting patterns. That the transmission is also shuddering is either a symptom caused by the same problem, or a result of a second fault in the unit.

It always raises suspicions when a problem crops up just after a vehicle has been serviced, and sometimes that suspicion is well placed. Have you double-checked the level of fluid in the transmission since it was flushed and refilled? Incorrect fluid levels can cause the sorts of problems you’re seeing. So can the incorrect grade of type of fluid used. You should check both these things first and move on from there if they check out okay.