99 Durango - rear end problem

From : mario leduc

Q: i own a 99 durango with the 360 v8 full-time transfer case and posi differential. i have approx. 75k miles on it and it has developed a string of problems that the dealer cant diagnose. the problem is intermittent and here is goes there is a chugging between 40 and 50 mph a vibration between 60 and 70 mph and occasionally a grabbing in the rear. the dealer ran the codes and nothing shows up. he also completed a tune-up for plugs wires cap rotor and purged fuel injectors. a soak of the combustion chamber was also done. the rear differential also had the lubricant replaced including the additive for the posi. as a last resort he pulled the driveshaft to check the u-joints and they seem fine. the problem is still there and seems to be worse when carrying more weight in the suv. the only word of advice from the dealer was to keep driving it until something gives!!!!! anybody seen this before .