Allison transmission vibration only in 3rd gear?

Just wanted to update this thread. Figured out the problem! It was the angle of the driveline, the one between the two axles. So the airbag height valve was messed with when we had a hitch installed and when they put the valve back it was incorrect and it gave too much air to the airbags which caused the driveline angle to be off. Found the correct ride height for the truck/suspensions and now it drives like a brand new truck should.