Are you a
Transformer? (Thanks to @smriti for the idea - R.I.D = Robots In Disguise)
Sometimes I look like a donut
Electrical transformers are often toroidal
Sometimes I look like a truck
Optimus Prime! He's a "Transformer" that looks like a truck.
I have many shapes so you know,
Both the robotic and electrical voltahe step up/down transformers come in many different forms
but i am no shadow.
No idea for this one.
Converting from one thing to another is my technique,
The robotic transformer changes shapes, and the electrical transformer changes voltage levels
but it is the very same thing that I seek.
The electrical form changes high voltage to low voltage or vice versa (it stays voltage). The robots also remain robots, just in a different shape.
Now you may search your garage,
though it won't be easy with my camouflage.
Robotic transformers are meant to blend in and look like normal vehicles.