1997 Ford F-250

I own a 1997 Ford f-250 HD 4wd with manual locking hubs. Recently when I lock the hubs and put it in 4wd the front wheels only have power for a couple of seconds then just spin freely as if there is no power going to them. I don't know whats wrong but would like to fix it. Any information on what might be wrong would be appreciated. Thank You
April 11, 2007.

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