Assuming the motor is basically sound, and so for the gear tower and final drive(s), it is possibly the dirty electrical wiping surfaces, but I would bet it has more to do with dry or gummy and generally polluted lubes. Grease does lose some volatiles over time that makes what is left behind stickier and less viscous. Lint, pet hairs, your body hairs, skin particles, house dust, insect parts, spider webbing.....you name it...it can all get wrapped around the axles and into bearings and bushings.
-Crandell