I have a 1987 Dakota 4wd with the 3.9L. I have owned the truck for about 2.5 yrs now and I have noticed a knocking sound coming from the back of the truck when I am going over 40mph coasting. When on the throttle there is no noise at all. When I put it in neutral going about 50-60 down the road the noise goes away. I checked my U-joints and they seem just fine a little slop maybe 3/16" worth of play. There are no vibrations or anything runs and rides smooth.
Can it be the back pig?(I think that is what it is called)
Thanks,
Brad

 

I would say check the rear differential, maybe you mean pumpkin?? I dunno. Anyways, I would take the cover off and inspect it, and at worst case you find nothign is wrong and changed your differential fluid.

Also, check the transfer case fluid while you are down there. Do you see any fluid leaks? Or wet spots around the shafts and seals?

 

If you have access to a dial indicator, check the backlash on the ring gear, it sound like there is too much. It should be between 0.006 and 0.008 inches of backlash.

 

That 3/16" of play, is that in the u-joint? If it is, I'd be replacing it before it breaks completely. If you have that kind of slop, the needle bearings in the cup have probably started to disintegrate. In short, there shouldn't be any side to side play in the u-joint.

 

When I say there is slop I mean rotating the drive shaft back and fourth. The U-joints do not have any slop in them. I will pull the cover off of the rear differential and inspect it. The transfer case does leak oil, but I top it off once a month or so it does not leak that bad.
Thanks,
Brad