We see in the film that the Wall-A robots move along grooved tracks.
However, we see at least one outside the ship in the end credits.
Since they travel on rails, how did they exit The Axiom?
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In short, we don't know. As you've noted in your question, the sole view we have of them outside the ship is a single image from the end-credit sequence (below) which shows a Wall-A lifting up another robot. The Wall-A doesn't move while we're watching it, so it's possible that it's being used as a static lift, installed into place and then moved afterwards by other robots or dragged on rollers by their human slaves.
Removing them from the loading bay would be a fairly simple operation since the bay door is larger than a single robot. You'd just need to use a few dozen SECUR-T bots to lift them down.