thats weird. i used to own an 870, it was a clutch model with mechanical 4x4. i believe its all geared, no hydraulics involved.....but its been 10 years and i dont recall all details on the 4x4 system.
Does the 4wd shifter feel any different? Can you verify whether the front driveshaft is still attached at both ends? Can you spin that driveshaft by hand when front wheels are off ground but back wheels are on ground, with 4wd engaged?