Here in NE, my limited understanding is that there is a slight difference in pay between package car mechanics and tractors. Top pay for mechanics is near top UPS driver rate, tractors is around $5/hr less.
Our mechanics are in an engineers union. And they just got done shafting their members because they mismanaged retirements. It used to be a gold 84 rule. Now it's 65. Apparently it's temporary...but in my experience you don't change a gold 84 to straight up 65 and make it temporary.
In my local they make the same as driver. The only down side is the lost there over time. The mechanics ask for overtime and the managers said transfer in to feeder or package.