The movementAnimations on the Jerry can end up moving it up out of the angled mount on the Mule (part slot json below) because movementAnimations are not performed in relation to the rotation in the part slot.

		{
			"pos": [-0.5625, 0.125, 2.9375],
			"rot": [0, 0, 75],
			"types": ["interactable_jerrycan"],
			"defaultPart": "unuparts:unuparts_part_unu_barrel_jerrycan",
			"toneIndex": 1,
			"minValue": 0.0,
			"maxValue": 3.0
		}

This picture shows how it is supposed to sit (before I got fancy with movementAnimations)

This is how it sits with movementAnimations

I'm a bit confused, what's going in the second picture? I don't see a jerrycan in the first one, or are those boxes the jerrycans?

They are mounted at an angle above the running board and under the body:

Fixed in V21.3.0.