The mip Mental Ray Production shader library in MR3.6 is "hidden" from the shader list at the moment for users of Maya 2008 as they havn't passed Quality Assurance from Autodesk as of yet.

Out of that hidden library, the 2 production MR shaders that "MAY" help your situation are called:
mip_motionblur and mip_motion_vector.

Keep in mind, however, that as they are hidden, that also means they are not supported as of yet, and there is no documentation for them yet either

Here is a link to Master Zap's Blog that explains this issue in more detail as well as ways to access them
EDIT:
OOPS forgot the link:
https://mentalraytips.blogspot.com/20...asures-of.html

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Otherwise, to blur in post you have 2 options:

1. render your layers keeping motion vector data.

-or the more common way-

2. Apply post mBlur to your loader/layer using a plugin that supports Optical Flow.

I'm not sure what compositor you are going to use so I cannot specify what to do in it.

However the most popular plugin for this is called

-ReelSmart Motion Blur (round $189 US node-locked)

Other apps have similar tools to achieve 3d motion blur in post such as:
-PFBlur for PFTrack ($330us)
-Boris Continuum Complete Sparks - has 2 included.
-Furnace also has the Kronos optical flow plugin.