95 Pathfinder grinding into 4-Wheel Drive when driving under normal conditions??

[jfh0006]

95 Pathfinder 3.0, 4x4 w/185,000 miles just started trying to engage into 4wheel drive when driving normal conditions around 30mph....I put the x-mission into neutral, then back into Drive...it seems to grind for a second, then drives normally.....it does it about every 10 minutes....took the front drive-shaft out....still same thing.....seems like grinding is coming from transfer case. Ckd all fluid levels...all appear normal....any ideas???thanks, Joe Hoerth