Man,you are asking a lot! I will do my best without being there to see and hear it for myself,and without a scanner!!
first,the gas leak,it sounds to me like either a vent tube or evaporative emissions hose/solenoid/canister has failed and leaking when the tank is full.
I have also seen where there is a hole rusted through the top of the fuel pump module.
if you cant see anything obvious from underneath with a light and a mirror,its time to hook it up to a smoke machine to find the leak.
I would venture a guess as to this is why the check engine light is on,the PCM has detected a leak when it performs the vacuum/pressure test,but who knows for sure until you hook it up to a scanner
now as far as the noise in the rear IF that is where its coming from,and only when you turn,either an axle bearing is failing,or there is something wrong in the differential.
cannot remember if that vehicle has a two piece driveshaft as there are not many of that model in my area,that said,if it does have a two piece driveshaft,its possible that the center carrier bearing support has torn loose and allowing the shaft to run in an egg shape circle,but that usually creates a noise under load and when decelerating.
I would be willing to bet that the noise is actually coming from the front end and you have either a bad CVC shaft which will make a ratcheting noise when turning,or the wheel hub bearing has failed,both of these will make a noise when turning,and when bad enough,will make a racket driving in a straight line.
crawl underneath to see if the boots are torn on the CVC,there will be grease strewn every where as well.
to check the front hub bearing,jack up the front end and see if you can rock the wheel top to bottom,any play here indicates a bad bearing if the upper and lower ball joints are good,it can be bad and not have play,so keep that in mind.
if thats the case,grab the knuckle or strut and spin the tire,if its rough you will feel it in that area.
if that vehicle has independent rear suspension,( for the life of me,I cant remember ) then yes,a bad hub bearing back there will give you the noise much like a front bearing will.
hope you figure it out!! let us know,and like I said,its dang tough to diagnose over the net!! so if I strike out on any or all of it,please forgive in advance!!!
