stumbling block/obstacle

[phorntita]

I need an advice from you whether these 2 words have any difference in connotation (or does it have any stronger meaning than the other one) when used in the same sentence like:
A lack of some qualifications can be a major stumbling block / obstacle to finding good jobs. or some other sentences like this
Your answer is urgently needed
I meant whether I can make a replacement / substitution with these 2 words when appear in any sentence.

[konungursvia]

They mean the same here, but obstacle is more formal and descriptive, and stumbling block is informal and metaphorical. I'd go with obstacle. They are similar but not always interchangeable.

[phorntita]

They are similar but not always interchangeable.

Please give me some examples of when it can't be interchangeable!
What about these 2 sentences?
1)A lack of some qualifications can lead to major stumbling block/ obstacle to your promotion.
2) A major stumbling block to your happiness is the fear that you may have cancer.
They could be interchanged here, couldn't they?

[konungursvia]

I just mean you don't always want to use more colourful, informal language, for social reasons. Semantically, they are equivalent.

[Sahmiam]

In my opinion, a stumbling block is something smaller than an obstacle, like a pothole you must drive around. An obstacle would be more like a tree fallen across the road. A stumbling block slows you down, whereas an obstacle stops you in your tracks.

[phorntita]

In my opinion, a stumbling block is something smaller than an obstacle, like a pothole you must drive around. An obstacle would be more like a tree fallen across the road. A stumbling block slows you down, whereas an obstacle stops you in your tracks.

Thank you for visualizing it.