What are the tube shaped structures on the back of the fuselage of this 737?
The aircraft is the Indonesian Airforce Boeing 737-2X9 “Surveiller” aircraft, used for Maritime Patrol. The fairings house Side looking Airborne Radar for matitime surveillance. From b737.org:


  Fitted with Motorola AN/APS-135(V) Side Looking Airborne Modular Multi-Mission Radar (SLAMMR) The antenna of which was mounted in two 16ft housings on the upper rear fuselage. This system could spot small ships at ranges of 100nm. 


Indonesia Operates three of them- AI-7301, AI-7302 and AI-7303.