Unknown rattle-ish sound 99 ranger.
 

New to this site,
I have a '99 Rangler XLT Super Cab 4x4 5 speed 3.0.
While driving i have this rattle-ish sound(maybe like a slinky or bolt rolling in a can or a bad bearing) coming from the front driveline area. I can really only hear the sound near other cars or near objects like a barrier on a freeway or close to a building. It only makes the sound while driving. And it is constant. I took the front drive shaft out and the sound stopped. Replaced the U-joint and put it back in and the sound came back. I've been all over the web trying to figure this out. The best I've found is either the driveshaft itself or more than likely the transfer case chain. But i haven't found anything identical to what I'm experiencing. Anyone experience anything similiar or have any ideas? I just bought the pickup a couple months back, it has about 120k on it.

120k isn't bad for a '99, I have a '99 B4000 with 126k on it :P

Is your truck in 2WD when the sound hits? Have you tried it in 4WD to see if the sound stops or worsens? What about reserve? I'm not sure what the sound could be, but playing around with it might help you pinpoint where it's coming from.

Its constant in 2wd. I don't what speed it starts at. As I mentioned i hear best near solid objects like freeway wall. That being said Ive tried it on 4hi a couple times but wasn't near anything that the sound would bounce off of well. I couldnt tell if the sound was present or not. It it wasn't it definately wasnt as loud. And I haven't tried listening in reverse. Its a good little truck, but the guy who had it before me spent a lot of time off road. The engine is sound, but the more I drive it the more little mechanical problems keep popping up.