2000 Jeep Cherokee 4WD problem

I have a 2000 Jeep Cherokee with 63K miles on it. About 4 months ago, I started having a issue with the 4WD. I was on a muddy road and began loosing traction control. I stopped and engaged the 4WD. The light came on and I could feel it engage. After driving for a couple of miles, it semed as if the 4WD slipped out. I heard a grinding/clicking coming from the transfer case. I stopped and tried to disengage the 4WD lever. It would not. I had to put the vehicle in park. From that point on, when enaging 4WD, the light will either not come on, or it will. It is hit and miss. Even if the light does not come on, I can tell the 4WD is engaged. I had a friend tighten the linkage, which was loose. He said that the light might be going. Today, I tried 4WD. The light came on but flickered if I moved the lever forward a little bit. Can anyone help? My friend mentioned that I should put the jeep in neutral before engaging 4WD, and not to do it on the fly. The slipping out of 4WD problem happen only a couple of times before I had the linkage tightened.

try adjusting the linkage....make sure that the transfer case is in 4lo(look at the tab on the front of the transfer case it should be inbetween straight forward and strait down basically however far down it goes) then loosen the nut on the linkage, hold the shift lever approx a 1/4 of an inch away from the very bottom of the plate with the positions labeled on it then tighten the nut down( most of the time this takes two people). if it still has trouble then the shift arms inside the T-case are out of alignment.
hope this helps