What was found
AIRCRAFT WAS GROUNDED: STRONG VIBRATION IN RUDDER PEDALS 2000' AGL BEFORE GEAR EXTENSION TO 500' AGL GEAR DOWN VIBRATION CONTINUED 20 SECONDS AFTER AUTOPILOT OFF. PEDALS LEFT AND RIGHT BOTH SIDES CAPT & FO MOVING 3/8" DISPLACEMENT. PUSHING ON BOTH PEDALS WOULD NOT STOP PEDALS FROM JUMPING AROUND, WENT AWAY AT 500' AGL. MAINTENANCE REPLACED THE ARTIFICIAL FEEL AND TRIM UNIT IAW AMM 27-23-41-4 00-001-A AND PERFORMED OPERATIONAL TEST OF THE ARTIFICAL FEEL AND TRIM UNIT IAW AMM 27-23-00-710-001-A AND 27-23-00-710-001-A-01 AND PERFORMED OPERATIONAL TEST OF THE RUDDER TRIM ACTUATION IAW AMM 27-22-00-710-002-A AND TESTS ARE OK.