I was able to complete the exhaust by increasing the chunk size and removing the jumbo tag from the problematic chunks.<br>
After getting the results from AdamC function, I found out the biggest chunk of that shard was ~148 MB of size.<br>
After figuring this one out, the first step is connect to the config database and change the max chunk size:<br>

    use config
    db.settings.save( { _id:"chunksize", value: 150 } )

The second step was to clear the **jumbo** flag from the relevant chunks.
This can be found in the MongoDB Documentation: [Clear jumbo flag][1] 

Once the chunks were not longer marked as 'jumbo' the balancer had no problem migrating them to another shard.<br>
Once all of this was done, I restored the chunkSize back to 64

    db.settings.save( { _id:"chunksize", value: 64 } )

As I do not want that all of my chunks will be as bigger. and when possible, I'll correct the hashed shard key on the problematic collections to create a more divisible shard keys.

Thank you AdamC, your initial guidance set me on the correct path to solve this problem.


  [1]: http://docs.mongodb.org/manual/tutorial/clear-jumbo-flag/