grinding is bad!!!! taking a to tight of a turn in 4wd will just make things pop and bounce but not grind and the gr5inding when in reverse doesn't sound good, sounds like your not engaging all the way like something is wrong with the t-case or whatever means these runners use to get to 4wd, i'm not an expert on the 4runners 4wd set up but grinding is never good esp. in the conditions your listing above.
are you sure it's in 4wd when your getting these sounds?
and again like said above using 4wd on dry pavment is a good way to break stuff, if your testing it out go very slow and don't turn the wheel to sharp, you will bind and hear some pops and chirps when cornering, but grinding should not be heard.