canister components behind differential on bottom of body

[need info]

I was changing my muffler tonight and I noticed a canister attached to the under body right behind the differential. its about 16 inches long by 4 inch diameter. I am guessing it is a vacuum type device for the gas tank. Can anyone tell me what this is.
Also I had to take the spare down and above the spare attached to the body is a rectangular box that is about 14 inches long and 3 inches high and about 4 inches wide. My guess is the same as above. Can anyone please tell me what this is? thanks for the information.

 

[adsm08]

Sounds like you are describing the evaporative emissions system. It is there to trap and store fuel vapor from evaporated fuel for future use.