Big clunk

[Unboy2000]

Good day; today while going downhill I heard a big clunk, it felt like it was coming from the transfer case; I kept driving but it happen again while I was breaking; a couple of km later It did again but this time I felt it in the steering while turning; I have been driving for over 30 years now and never experience something like that; It almost felt like a CV joint was coming apart. Or like I drove on a log...look under the truck and don't see anything obvious. Both wheel spin without any noise or loose. When this incident happen the truck was lacking power and took forever to get up to 80km/h almost like if the truck was in 4X4 any idea ?

6.2 GMC 4X4 1992