I noticed today that only one of my trim cylinders is working. With both of them disconnected from the drive only one moves when activated. I'm assuming this means the trim cylinder is defective. Are these things rebuildable? If so, how hard is it to do the rebuild?

No need to rebuild yet . When they are disconnected and one is looser than the other , that one will move first and most. When there is the resistance of the drive on them , they will probably both work