At that time she was working in a PLA unit.

At that time she was working in a PLA unit.
At that time she was working in a PLA unit.

At that time she was working in a PLA unit.
那一段时间,她在一个中国人民解放军单位工作

那时，她在解放军某部工作。

那个时候，她在一个解放军的单位工作

反正就是这么个意思了