4WD Not Working

I purchased my 02' Envoy 3 weeks ago, I live in Canada and we have a foot of snow on the ground. I purchased it while it was engaged in 4hi, I never tested the 2hi, only 4AWD and 4LO everything disengaged and re-engaged fine. We had a cold front early this week with temperatures as low as -55 with the windchill. My fiance drove the truck to school and while pulling out of the university campus the truck stalled out and threw the code ABS failure and service braking system. She would restart and as soon as she would put it into drive and accelerate the truck would die out. Once I got there I shut the truck off as it would idle and run fine, put it into gear and it clunked out, I shut it off turned it back on and it illuminated the check engine light but still ran fine, I put it in gear and it clunked out. I repeated the process shut truck down re-started and the oil pressure low light came on turkc still runing fine, the oil pressure gauge read normal I checked the oil and the level was fine. I turned the truck off and after re-start I could here the transfer case actuator engaging like I was switching it out of 4HI, So I started playing with the dial, turned it from 4Hi to 2Hi no codes or anything came up at that start-up and I put it into drive and it drove with no issue what so ever. We drove it home across the city in 2HI and I pulledit in my heated garage as the dealer suggested there must be frozen sensors with all the codes it was throwing. The next day I pulled out of the garage put the truck in N and placed the switch to 4HI heard the actuator engage and put it into Drive and away I went. As I pulled out of the Bay I spun a 180 and realized the 4x4 wasnt' engaged. I came home logged on searched for any related problems online. I took it to the mechanic and he suggested it was the 4x4 actuator located on the passenger side of the differential. He gave me a part number I went and got the part and replaced this afternoon, still no 4x4. It makes a slight grinding noise while activiated and it sounds like it's coming from right behind the actuator switch. Could this be the whole assembly behind the actuator? I believe they usually have a fork and colar? Maybe these are worn out? Could it be the front diff? Any suggestions? Anyone had this same thing happen?

Sorry for the half a book read, i'm just uber peeved right now.

Thanks,
HH