Mine had much the same sound it took ford a few time over a period of a year to diagnose that it was a gear in the transfer case on ice it would clunk and the power would transfer to the rear wheels.

Hope this helps.

 

It would seem more likely for the sound to be coming from the transfer case than the CV joints. On the newer models, the driveshafts turn all the time, so the CV joints are turning whether or not you're in 4WD.

By the way, if you hear this sound while running in 4WD on hard surfaces, then that is the cause of the noise. You are NOT supposed to do that. It's bad for all the parrts involved. You're only allowed to do that if your vehicle has a center differential, which the Ranger does not have.

 

My 06 ranger has the same issue. Clunking every 10-15 secs in 4h and every min or so in 4lo. Only when I'm on the throttle. No noise when coasting. I put the truck on 4 jack stands and ran in 4h and 4lo. No clunking. Checked the front drive shaft and looks ok. Thought it could be the shift motor so I replaced it and the clunking is still there. Thinking of rebuilding the t-case.