Zac Bert

Good evening, My 01 2500hd has recently developed a 4WD issue. If I hit the 4WD button, it will clunk in as it normally should, except 4WD doesn’t work. If it is in 4 while driving, it doesn’t grind, but the second there is a need for 4WD (the rear tires hit a ice patch on a hill), only the rear tires spin and there is a nasty grinding noise. I at first thought it was the 4WD actuator, but replacing it did not fix anything. The noise does sound like it is coming from the t-case. When in 2wd, there is very little grinding, if any. I’ve noticed it grinding once, but thought it was a heat shield rattle. Has anyone come across this or is there a known cause? Thanks in advance. - Zac