Transmission & torque converter
Every gear engaged in both directions, under load, from cold and then warm. Shift quality, slip under load, stall behaviour, oil condition and smell, and any delay between selecting a gear and getting it.
Why it matters. The most expensive single component group on a backhoe and the least visible. Burnt-smelling transmission oil or a gear that engages slowly is a finding that should change the price, not a quirk.