[pie™]

I am living this problem with my new to me 2000 blazer that I bought the day after Thanksgiving. I took it to my mechanic initially to have him give it the once over in the post 30 day purchase period and he found no problems with it. The first time I went to use the 4WD (light winter this year of course) I found that the 4WD didn't work. Back to the mechanic who found a hose detached. He hooked it back up, test drove it and voila- the 4WD worked. Alls well, I thought. A month or so later - it started that hum you speak of - at speeds from 10-30 mph, sounding like it was coming from the right passenger front. Back to the mechanic who couldn't get it to duplicate but thought the 4WD was sticking. It clunks into 4WD -so he messed with it and proclaimed that he got it to "unstick". To make matters worse -its got snows on the rear and all seasons on the front and I've talked to both my mechanic and my tire guy about that -and they've said, "its not great - but not going to do any damage" Boy were they wrong.
Within 2 weeks now - its gone from making that hum intermittently now and again - and I can get it to stop by either pulling over, putting it in park, pushing on the 2WD button - or some variation - to nothing would make it stop and by Sunday this past week it did the hum for 2 days straight every time I took it out. Finally, I got so frustrated, I put it in 4WD and the humming stopped - but the clunking began. CLUNK, CLUNK, CLUNK, all down the road. After back and forth with that for a couple miles, I got so frustrated that I pulled over and just pushed 2HI, 4HI over and over - and then- like magic - no hum, no clunk.
Got it into my mechanic today to do some routine service and to find out what is ALSO making it creak like a rust bucket (new ball joints now) - and they're thinking differential - and oh by the way, they say, you need to have 4 new tires- all the same tread size - because that could be half your problem -which could also have caused major damage because by Sunday -the humming was starting to sound like a grinding and was from 0-40mph that I could hear it.
Bigger problem - they're keeping the car and going to test drive it tomorrow after ball joints and new tires- and if they can't duplicate the noise -the mechanic says theres no way to diagnose the differential as the problem without tearing the car apart and that could cost about $900 - and a new transmission seal, new u joints- and in the end -it could end up NOT even being the differential - they just don't know.
Is there a way to diagnose it as the differential - or could it be the front axle? Or I've heard about the actuator - or the switches- ?? Any suggestions on this would be helpful here. I'm afraid tomorrow my mechanic is going to test drive it - not get it to duplicate either the thumping in 4WD or the humming/grinding in 2WD - and proclaim it fixed with new tires- and I'm going to have major damage driving it when it is indeed something to do with the differential.
Thanks