CRJ-200 pilot reported abnormal airframe shuddering resulting in a diversion to the nearest suitable airport for landing.

Narrative

Ferry flight. Normal preflight walk around; normal push and start up. Nothing abnormal. Take-off and climb out standard. Climbing through 28;000 ft. for 31;000 ft. as Pilot Flying (PF) the Captain and I noticed and abnormal shudder through out the airframe. Originally believed to be turbulence leveled 31;000 ft. at Econ speed 253 kts. Shudder still persisted. Captain and I came to the conclusion that this was not normal or anything associated with turbulence. Requested lower 29;000 ft. and slowed to around 240 kts while monitoring engines and flight controls; all normal. Shutter still persisted through 29;000 ft. and again requested lower 25;000 ft. Captain noticed rudder slightly out of trim and I attempted to correct but correcting made shudder worse.Adjusted trim back to original position. Safety of the flight was then called into question and decision was made to divert. ATC and Dispatch was notified of our intentions to divert to ZZZ1 and [request priority handling]. Shudder was still noticeable throughout the descent and approach to landing however less pronounced. In the descent I made the decision to exchange the flight controls over to the Captain for the approach and landing. Standard approach to landing was made to ILS XXR without further incident. Normal taxi; park and shutdown was completed without further incident. Post flight duties were completed and appropriate team members notified. The cause of the abnormal airframe shuddering was unknown.