Learjet 75 First Officer reported autopilot and trim issues during flight. Excessive aileron trim was required to maintain level flight and the flight crew diverted to make a precautionary landing.

Narrative

On Day 0 at approximately XA:35 AM PST; Person A and Person B were climbing through FL280 for FL 410 after departing ZZZ. a master caution illuminated for the autopilot holding lwd. Person B opened the QRH to the appropriate checklist while Peron A disconnected the autopilot. The malfunction cleared and the autopilot was re-engaged. Knowing that Aircraft X had been rained on overnight; it was assumed that this was a sensor issue from the moisture. During troubleshooting; the malfunction occurred a couple of more times. As the aircraft was hand flown; it was noted that there was a slight roll drifting to the right. Person A even asked Person B if all of the passengers were on the right side of the aircraft; slightly in jest; but that was the sensation that was felt while flying by hand. One notch; and then two notches of left aileron trim input were given. The autopilot was re-engaged one final time and the same master caution appeared again (this was the only specific malfunction annunciator that ever illuminated from the initial incident through the completion of the flight). This time; while hand flying; Person A noticed that the rate of roll was increasing and more left aileron input was required to maintain level flight. Airspeed had not been changed. Per the QRH; the aircraft was going to continue to be hand flown while maintaining a speed below .78M. ATC was advised and the destination changed to ZZZ1. Person A began a left turn and descent while Person B ran checklists and coordinated with ATC. It should be noted that at no time were any of the flight controls frozen. While descending through approximately 16000-10000 ft; a noticeable bump was felt in the yoke as if something let loose or had disengaged. Control forces remained constant; however; with approximately 30 degree left yoke and 2 notches of left aileron trim. ZZZ Center originally cleared the aircraft to land on the North Runway XX while descending visually through approximately 7000-8000 ft. over the mountain range to the East of ZZZ1. Person A was not comfortable slowing the aircraft enough to fly a traffic pattern to XX since they were positioned slightly to the north of the airport. Person A did not want to fly at slower speeds and make more turns than necessary due to the unknown nature of the flight control malfunction. ZZZ1 Tower authorized a landing on any runway; thus; an approach and landing was made to XY. Weather conditions during the descent were VMC and ZZZ1 reported winds calm while on final approach. Note that during the descent; the spoilers were engaged between half and ¾ deployment with no change in required control inputs. A flaps 20 landing was utilized to prevent making unnecessary configuration changes at a lower altitude. It should also be noted that the control lock was in while the aircraft was on the ramp at ZZZ. The deflection of the ailerons was to the right while the control lock was in place. The landing was accomplished smoothly; however; it was overweight at approximately 20200 lbs. There were no injuries to the five passengers or pilots. No damage or irregularities were noted on the post flight inspection.