V + 不了
This potential complement usually describes something that you generally can't because of an external factor out of your control. 没有办法!
我的腿受伤了,所以我走不了 "I can't walk because my leg is injured"
I didn't decide to get injured (did I?)
这数学问题太难,我帮不了你 "This math problem is too difficult, I can't help you"
I'm unable to help you because I lack the necessary skills.
这是非法的,我做不了 "This is illegal, I can't do that"
I can't do that because the law forbids me to do that
V + 不动
This instead describes your own physical inability, and not an external hinderance.
When talking about physical feats though, you might think: "Wait, if I'm physically unable to do something, it's not really under my control, is it?". Well technically it is:
我累得走不动 "I'm so tired I can't walk"
In this example I can't walk because I'm too tired, but I could theoretically force myself to keep going (and collapse shortly after).
Therefore here:
这个书架,我们俩抱不动
The two of us can't physically carry the shelf, but there's no external factor stopping us from doing it anyway (and get crushed afterwards...).