Clunk sound in front end

When in 4 wheel drive and driving 55 MPH sometime slower i hear a clunk sound in the front end it is not constant. about every 1/4 mile or less it will pop or bang almost like the hubs are poping out. I thought it might be the front drive shaft u-joints where bad like on my 85 ford bronco II but they are ok could any tell me what the problem might be thanks

My 75 ford did that and it was the hub, You should be able to hear if they are turning smoothly if you put it on a jack stand and manually turn the tires. Mine just rattled. My father inlaws 79 GMC did the same thing.