Correct me if i'm wrong but If you have Spider gears then you don't have a G80 locker. Spider gears is an open differential and if the gears on them is wearing down then they are having a hard time grabbing each other.

G80 (ignore the yellow arrow)


Spider gears

 

The G-80 also has the side & spider gears. If you look closely at that top pic, you can see them there.

 

Starting to think its not the diff. I was out driving and was going slow (~10-15 mph) over a long string of clackers. Almost every time the rear wheel hit a clacker, I got the clunk. This was going straight, so the diff shouldn't be trying to lock/unlock. Weird, but damn annoying. Gotta get this shhitt figured out.

 

See if someone has some spider gears laying around and swap the G80 with the spiders and see if it still does it.
If it's doing it as an open diff then you definitely have other issues.

 

do you have something else going on, like exhaust hitting a shock, or leaf spring, could also be a pad missing in the leaf pack and the springs are hitting together. If it is up front, like you mentioned earlier, check the steering stabilizer, ball joints, hub bearings, Idler/pitman arms. All of those up front will clunk, Also do you have a 1 or 2 piece drive shaft the carrier bearing/rubber bushing could be busted.

Just some thoughts to look at

 

I had it up in the air today for a tranny service and to look for the obvious. Everything looks good. Gave everything the shake test...tight. Good point about looking at the pads in the leafs, I'll have to check that. I have the 2 piece driveshaft. The clunk is definitely in the rear, but there is a pop once in a while up front when backing up and turning. EFF'ing mystery...