fabtech4x4

No other good way to explain it. i have a 2005 lariat 4x4 with the switch on the dash. Was out doing some driving today in the snow, went to switch into 4wd and it didnt click in... but the light came on! and stayed on. so i switched it back to 2wd and got up the hill... got on a level surface.. got up to about 10mph and tried to switch to 4wd. again.. light came on...no 4wd. but it was a slight grinding sound off and on and would thump every 3-4 seconds or so. I figure this is probably in another post... i just couldnt find it. Any ideas???? Im lost