Hi,
Here is a video which shows the dis-assembly of your impact driver.
https://www.youtube.com/watch?v=TzD6G99tl9U
I think that the gear assembly, shown being removed at 1:25 minutes into the video may be jammed and the spring cannot function correctly.
Here is a link which shows the exploded parts view of a Rigid R86034 X4 impact driver.
http://www.ereplacementparts.com/ridgid-r86034-18v-lithiumion-impact-driver-kit-parts-c-7929_8253_167778.html
Hopefully this is of some help