If the attributes of the object with the dynamic name are always the same, you could try to return the json string in your REST call, do a manual replacement, so that the names are always the same. And then apply the mapping.
Maybe there are better solutions, but that's my first thought.
The property names "abc" and "abc2" are not known in advance, so they can't be used in an import mapping.
There are two solutions:
1 - Can you change the JSON format? If modelObjects were an array instead of an object, you could parse i. The property values are available in the uid property.
"ServiceData": {
"modelObjects": [
{
"uid": "abc",
"className": "className"
},
{
"uid": "abc2",
"className": "className"
}
]
}
2 - If you can't change the JSON structure, you need a custom java action that converts the json into the format above.