Transmission will not engage "drive" or "reverse"

Out of the blue with no prior issues, my car's transmission slipped after leaving a red light. Will engage "neutral" and "park", but just rolls through "drive" and "overdrive" like it is not even catching it. Fluid is fine and does not appear dirty, but have not looked at pan yet. Linkage appears to be moving while shifting, just not catching though the nut and bolt on the linkage are very slightly loose. No computer codes thrown or sensors. Sound like a pump or torque converter maybe?