09 GMC no 4wd, makes clunking/clicking noise

[Kylesdgmc]

Just bought an 09 GMC and when I tried using the 4wd it makes a clicking/clunking noise in the front axle and won't engage. What's weird is the 4wd works fine in reverse, but when going forward it kicks out of 4wd and acts like it's slipping on gears or something in the front axle. Really sucks spending as much money as I did on a nice 4wd pickup that the 4wd won't work on. I'm fairly mechanically inclined and did some looking into it. The transfer case locks up like it should and the front actuator is good. I jacked up the front end and put it into 4wd, spun the tire and it locked up one way, and when I spun it the other way it spun fairly freely along with the clicking/clunking noise. Any ideas would be greatly appreciated.