OK, two questions for you guys. Over the last month, there has been a noise coming from the drive shaft or the rear diff. I do not think it is the u-joints because from a stop, I get going and maybe 5 feet after starting there is a clunk, clunk, clunk noise that is hard to describe. I can barely feel it in the floor when it happens and everything is smooth driving from that.

Also, I changed the rear diff fluid a few months ago and noticed what is in the pic yesterday when taking a quick look underneath. I see many 4runners every day and never see this. I used a new crush washer and it is tight. Also, there is no dripping of any sort and it is just like oil and dirt. Is this normal, or should I pop it open?

 

stethoscope
jack stands
assistant
listen to diff up close

 

It's time to check your diff fluid level. I hope it's the u joint because if the diff is making that noise, it's too late to save it

 

I have the same problem but mine is more of a click than a clunk, I determined mine to be the carrier bearing