While I admire and support the effort to save money, one still has to consider what the shock was designed for.
There is a huge difference in automotive shock/spring design and Mule design. This difference can affect ride, steering, stress on frame members and safety.
I can understand exploring many option IF an OEM shock was not available, but they are available. They are costly, but what is the cost of an aftermarket shock? Will you save more than $100 per shock? Is it worth it? I say no. Safety should not be compromised.
Even if an automotive coil over shock is found, my guess is it won't fit due to the much larger outside diameter of the spring.
My two cents worth? Stick with OEM.