A trial system for an implantable joint replacement includes a guide assembly having a baseplate and a guide plate, the guide plate translatable relative to the base plate along a straight path while directly connected to the baseplate. A first adjustment mechanism may selectively control the translational juxtaposition of the guide plate relative to the baseplate. The guide plate may further be rotatable relative to the baseplate around a rotation axis. A second adjustment mechanism may selectively control the rotational juxtaposition of the guide plate relative to the baseplate, and can lock the plates together. The rotation axis may be medially offset from the center of the guide assembly. The guide plate provides in situ positionable cutting guidance for implantation of prosthetic joint components. The system may further include an articulating insert trial mounted on the guide assembly to determine preferred joint alignment and/or optimal positioning of the guide plate.