Reproduction steps:
1. Open the attached project;
2. Open the only one scene and select the Cube object;
3. Observe option in the Inspector;

Expected results: Joints are supposed to have a “Mode” dropdown option under “X Drive”, “Y Drive”, “Z Drive”, “Angular Drive X,Y, and Z” , as well as “Slerp Drive”.
Actual results: the Mode dropdown is not included here.

Note: The Mode option is mentioned in Unity docs of Configurable Joints, from 5.4 to 2017.1.

Reproduced on: 5.4.4p3, 5.5.0f3, 5.6.1f1, 2017.1.0b5.