Sorry, this is a bit lengthy, but better lots of info than not enough...
I have a 2000 Escalade, that I bought used about 2 years ago. It'* been well taken care of, and has relatively low miles (about 140,000) for its age.
Anyway, I was driving home in a typical small residential area, going up a slight incline, coming up to a stop sign - so only moving at about 15 mph or so. There was a LOUD bang, like someone whacked my undercarriage with a sledge, and from then on, a regular interval clunking until I stopped. It was similar to the clicking you get from your trans if you put it in park before coming to a complete stop, and it clicks a couple times before it catches and stops...except LOUD, like STUPID LOUD, I had to yell over the noise to my wife. People walking on the sidewalk 2 blocks away were turning around looking to see what it was.
Being partially disabled from a work back injury I can't really crawl around on the pavement, so I asked my wife to hop out and watch underneath to see if she could spot anything out of place while I slowly moved the vehicle. She saw nothing unusual visually. While doing this, I noticed that once I got the vehicle moving, if I put it in neutral and let it roll forward, the insanely loud clunking would stop, and it rolled fine. Also, at one point, I coasted to a stop (remember it was slightly inclined), and I tried to let it roll backwards to see if anything different happened. Oh yeah - it STOPPED instantly, like I had backed into a brick wall. Still in neutral. After that, I did not try to actually put it in reverse to back up, though now I think I probably should have tried it.
So, I was only 3 blocks from home at this point, so I just got up a little speed in gear, then put it in neutral to coast most of the way without the deafening noise. Parked it in the driveway, and there it still sits. I was already thinking of getting a different vehicle, mostly for better gas mileage, so a couple days later I did just that.
The engine ran and sounded perfectly normal during this, and I checked the trans fluid when I got home, the level was fine, no discoloration, no funky smell.
I'm trying to decide if it'* worth it to me to fix the thing, or just sell it as is. In my area, the bottom has completely dropped out of used car prices, so it doesn't make sense to spend a lot. Even though the body is in great shape, and the custom 20" wheels were about $1800 when purchased new, I still don't think I'd get much for it all the way it is. Basically, I'm hoping someone here has encountered a similar issue, so I can make a better informed decision on what the cost might be. Plus, I live out in the boonies, so just getting it flatbedded to a repair shop won't be cheap.
Thanks much for taking the time to read all this!

If you could get someone to get your Escalade up on jack stands so all wheels clear the ground and peer under the vehicle while it'* engaged in both forward and reverse gears it may help to narrow the problem down. I suppose it could be the transmission, but judging by the symptoms you presented I would guess transfer case, either one would be a spendy fix.

Broken front drive shaft?

Bad front differential?