Thumping noise in drive train.

Hello, I have a 1998 2 wheel drive Tacoma with 320k rather hard miles on it. For the last couple thousand miles have been towing a 4000 pound trailer and a thumping noise has developed in the drive train and has gotten worse. The thumping frequency is proportional to the speed and seems to be the loudest when coasting or decelerating. At 20 MPH there is about two thumps per second. At that speed I calculated the wheels are rotating at about 4-5 RPS. Here is what I have checked so far.

1. The universal joints seem to be tight and in good condition.
2. When I put the truck up on jack stands and run up to 30 mph there's no noise.
3 I changed/checked the transmission gear oil and it looked fine.
4. I changed/checked the differential gear oil and it seemed like there was a fair amount of metallic sludge on the drain plug magnet. (About the volume of one kernel of corn). This seemed excessive but after checking online it may not be out of the ordinary. The oil had never been changed and the sludge was very very fine.
5 When driving on a dirt road it seemed the noise was much less. I had just replaced the tires and it seemed like they were cupped somewhat so I rotated them but this didn't help.
6. When decelerating, starting at about 5 mph it seems the thump can be felt in the steering wheel. It's slow speeds it almost seems like the thump could be more of a quick grinding noise.

Any ideals here? Thanks

 

Too much backlash in the diff due to 300,000+ miles on same gear oil. The shims have turned to dust.

 

^^^ agreed... grab the yoke at the front of the third member and wiggle it up/down in and out.

 

i can be your third members is just worn out it happens with you tow alot. same thing happened to a 4runner i have i towed with it and the rear got worn took out the third shimmed it problem went away