After having difficulty finding the step file of the individual gearbox on the Andymark website. I downloaded the 2016 KOP chassis model and used that to obtain the model of the gearbox housing.
If you a wanting to 3D print it ( which I am assuming as you asked for an stl file), you might need to do a few modifications to the model to make it more suitable for printing. I rotated the housing so that the area of the housing where the CIMs are connected will be on the print bed but you may still need to modify it depending on how good your printer is at doing support material. I have uploaded the .stl, .step and the solidworks part file to enable you to do this.
https://grabcad.com/library/tb-housing-1
If you need help modifying it just send me a PM or reply below.
The .stl file may be 25.4 times larger then it should be because being an Australian, my SW is set in mm not inches so be careful.