i could use some help... i have a 2007 ford escape hybrid 4whl drive with aprx. 35k miles on it... it is run perfectly until about a month ago... when i put it into reverse there is a loud "clunk" noise coming from the rear of the car... i took it to the ford dealer and they have had the car for about a month...when they disconnected the drive shaft the noise went away.... so they replaced the drive shaft... the noise was still there... then after a lot of dialog with ford hotline assistance they replaced the rear end... (not the axles or cv joints) but the whole rear end.... and the noise is still there... they have now said that there is nothing more that they can do! and that i must wait until they have another escape hybrid 4 whl drive in the shop so that they can compare the cars and see if the noise is present in the other car... but.. they have not been able to locate another car... and the noise does not sound good... it is a loud clunk... and it just started about a month ago... can anyone please help me with this?

p.s. after reading some of the dialogue between the dealer and ford they suspect the PTU? and they also said that if they command the rear end anywhere from 0% to 20% the noise goes away... but if above that the noise is back... and when they command it to 100% then the noise in there both in reverse and in forward....