2006 Nissan 4wheel
I bought this car used last July for my trip to baja. Heard small knocking noise coming from under right rear end…it got worse. three different mechanics could not find the problem…one was sure it was the differential and replaced this…not cause…then said it was a slightly bent axle and sent me on my way to drive solo 4000 miles to mexico,said the car would make it 
he will fix it when i return meanwhile my warranty expired…knocking is really loud now…question: how does he know if its an axle?if it is didnt I ruin something by driving it so far?? i think he doesnt even know its this…what test can I have done to determine this is the problem?
If the car was still under warranty at the time of the problem, your BIG mistake was taking the vehicle to anyone other than a Nissan dealership. Even if it had to be towed, that was the place for any problems to be addressed.
Now that the warranty has expired, Nissan will disavow any responsibility, and–more than likely–so will the independent mechanic to whom you mistakenly took the car.
All I can suggest at this point is to seek out a competent independent mechanic.
The warranty was a 6 month 6000 mile warranty which expired just after the third mechanic looked at it…i failed to say one of the mechanics was Nissan they could not find what was causing the noise either…is there a test or something that shows a bent axle…I was looking at nissan problems and one was axle bearing plate??? could this be causing the knocking …I hear it going slow when I accelerate? and is slightly makes a scraping/screach noise when I back up…since I was aware of it a little after I bought it (but didnt think it important at the time) should I go back to the seller warranty no good but nobody can find the cause and I had witnesses early on.
Drive it until it comes to a screeching halt then fix whatever is broken…