I was having the exact same problem with my Gen 1 Pajero. So I got all my axles, hissa, diffs checked. There was no problem with any of them. If you do not have this issue when your are in 2wd, chances are that your transfer box (4wd box) needs to be replaced. There is a chain inside the box that goes bad, as a result, the chain rails get play and start slipping. This fix is happening right now as we speak on my Pajero in G7 in Islamabad. I will find out more tonight, and update as soon as the transfer box is replaced. According to two mechanics I have shown my ride to, this is a common problem in aging 4wd vehicles.
Also, as others have mentioned, if your front diffs aren't locked, you can be in 4wd with no issues on tarmac. Also, if you are out and about on an offroad trip, and lose your rear axle, you can always switch to 4wd to bring your car home.