積む and 下ろす: the loading position and the source of lowering
Read loading onto a cargo area and lowering from a shelf with their locations. Distinguish where the object is placed from where it is lowered.
Where the goods are loaded
積む, read つむ, means stack or load. 荷台に段ボール箱を隙間なく積んだ。 says that cardboard boxes were loaded tightly onto the cargo area.
荷台に is the loading position. 隙間なく describes this loading arrangement, not a requirement for every use of 積む.
The source of lowering
下ろす, read おろす, means lower or unload. 棚から重い箱を下ろすときは腰に気を付けて。 asks someone to be careful with their back when lowering a heavy box from the shelf.
棚から gives the source of the lowering. The request for care does not report that the box has already been lowered.
A different object of 下ろす
タクシーは駅前で客を下ろして走り去った。 says that the taxi dropped off the passenger in front of the station and drove away.
The passenger is the object, and both dropping off and leaving are reported. Preserve the object and location in goods comparisons without restricting 下ろす to boxes.
In 荷台に箱を積んだ。, where are the boxes loaded?
Show the answer and why
Onto the cargo area. 荷台に gives the placement location, unlike から marking a source such as a shelf.