A B737-300 RETURN TO FIELD DUE TO LOSS OF FO'S PITOT STATIC INSTS; AIRSPD AND ALTIMETER.

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ON AUG/XX/96; ACFT ARRIVED AT ZZZ ARPT WITH 2 WRITE-UPS. ONE WAS GND PROX WAS INOP. THE OTHER WAS THEIR L IRS HAD A MOTION FAULT. I RPTED TO WORK; NEXT SHIFT. SINCE I WAS AN AVIONICS TECH/MECH; THE LEAD ASKED IF I WANTED TO WORK THE WRITE-UPS. I DETERMINED THAT THE IRS MODE SELECT UNIT APPEARED TO HAVE A LOOSE SHAFT. SO I REPLACED IT. AS A PRECAUTION I ALSO CHANGED THE L IRU. IT'S LOCATED ON THE R SIDE OF THE E&E COMPARTMENT ON THE B737-300. I PERFORMED A LAND VERIFY TEST PER FMC. BY REPLACING THESE ITEMS THE GPWS WAS ALSO FIXED. THE FLT LEFT. SHORTLY AFTER TKOF THE ACFT RETURNED TO THE ARPT WITH THE FOLLOWING WRITE-UP. SHORTLY AFTER TKOF; AUTOTHROTTLE OFF; FOLLOWED BY GRADUAL LOSS OF AIRSPD AND ALT INDICATIONS ON COPLT'S INSTS. NO FLAGS ON AFFECTED INSTS. CAPT'S INST OK. STALL WARNING CAME ON UNTIL FLAPS TO UP DID NOT SELECT. ALT PITOT STATIC. UPON INSPECTION I FOUND #2 DADC STATIC LINE LOOSE. I RECONNECTED THE LINE AND PERFORMED A PITOT STATIC TEST PER MAINT MANUAL. I ALSO PERFORMED A DADC SYS CHK PER FMC. ALL INDICATIONS WERE WITHIN LIMITS. I FEEL IT'S POSSIBLE; DUE TO THE LOCATION OF THE DADC #2; THAT IN REMOVING THE IRU; I COULD HAVE BUMPED THE LINE CAUSING IT TO BECOME AJAR. I'M NOT SURE HOW TO PREVENT A RECURRENCE. I HAD NO INDICATION IN THE COCKPIT OF A PROB. THE LAND VERIFY TEST CHKS THE DADC; BUT NOT CONNECTING PITOT OR STATIC LINES. THE ONLY WAY I CAN THINK OF WOULD BE TO INSPECT ALL PITOT STATIC LINES WHEN THE #1 RDU IS REPLACED. ON THE B737-300'S THE DADC SITS O THE TOP OUTBOARD R SIDE.