help with tow vehicle issue

I have an 04 Chevy Suburban z71. 60k miles. I was driving today and came to a stop sign. Stop then start to go and loud grinding. and car won't move. So I put it in park, fine. Put it in Drive, immediately grinding won't go anywhere. I don't know cars/trucks at all. I looked under the car while my wife put it in gear and dive shaft is turning, but its like its no longer connected to my wheels or something. Had it towed. any ideas.

Have someone put it in drive with the brake on . Look under the truck [ do not get under the truck]
see if the drive shaft is turning. If it is you have differential problems. If not see if the grinding is coming from the trans.

Did the check engine light come on? Do you you have onstar? If so, you can press your onstar button and they will tell you what error codes are generating. Saved me out when I got stuck in palm springs.

drive shaft turns with brake on.

no engine light, no on star

Sounds like you either broke the u-joint (though that should be pretty obvious seeing the driveshaft not attached to the rear yoke) or broke something in the rear differential. I'm leaning towards the rear diff, and not going to be a real cheap fix.

Agree, either you snapped the u-joint in one of the driveshaft yokes, or possibly broke the pinion gear in the rearend. If it's a 10-bolt rear end and not a 12-bolt I'm betting pinion, been there done that. Probably ~1500 to get rebuilt with new gears assuming you haven't ate the axleshafts too.

Any chance it was in 4wd at the time?