"Let me abo's go loose" simply means dismiss my field hands. The song caused some controversy at the time, and I remember it was banned from the airwaves in some (Asian? ) country as a result of this verse, which was in dubious taste even allowing for the less politically aware climate of the 60's.

One Guest above wonders why the kangaroo needed to be ties down in the first place.

My understanding is (and always has been) that "kangaroo" in this context is allegorical, being symbolic of the dying stockman's male member, which he is afraid might continue to flourish priapically even after death just as it did when he was alive, leading to potential problems with the fitting of the coffin lid. Therefore he beseeches his closest friend to take appropriate precautionary measures.

Murray