Can a bad PTU cause the car to feel like it is VERY heavy in the back end?

What is your question?

I have a VERY well maintained 2007 Edge (AWD) but have recently started smelling something burning. The vehicle also feels very heavy in the back end; almost like I'm towing something. I've not seen any kind of link, but I do get the smell. I've taken it to the dealer twice and have been told both times that there is nothing wrong with the vehicle. I've driven this car for 7 years and know that something is wrong; I just don't know what it is.