For the last week or two, I've noticed something I can only describe as occasional "lash" or a "clunk" from my 4x4's back end. It's like something gets out of sync and needs to "skip" to catch up. The Dakota has about 25K miles. I seem to recall reading somewhere that this type of behavior could be from old fluid in the transfer case or differential (I forget the specifics). I've also noticed a drop in MPG that did not rebound when I had the oil changed recently. When should the fluids be replaced? Does it sound like the fluid is the problem, or something else? What's involved in switching to full synthetic and what brand would you all recommend? Thanks!

 

I changed my rear diff and front diff at @40,000 miles with synth. 75W140 Mopar gear lube. I also did the transfer case (Mopar ATF +4). I tow with my dakota so I went with the 75W140 synth in the rear diff. If you have limited slip in the rear you'll also need a 4oz bottle of friction modifier. Do not forget the RTV sealant for the diff cover.

I stuck with all of the Mopar fluids for this. There are many who have used Amsoil etc .. depends on how much you want to spend.

As for the issues you are having , I did not experience any of them prior to my fluid change.