Chevy 1997 K1500 Trans Problem

The computer set code PO187 and it says that the 4wd switch is shorting to the battery. Can anyone tell me what this means. The truck will only run in 1st and 2nd, no 3rd and 4th.
thanks
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Ok. I was just going to ask if this switch is on the outside of the trans or on the inside?
Thanks
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Are you sure it is not p1870?

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