Spare tire question

[FreckyB]

I have an 08 Z71 Silverado with 18 inch wheels, my current 285/65 R18 Terra Grapplers are in need of a change so I ordered some Trail Grapplers of the same size, my spare tire on my truck is on a 17" rim and stock size so I'm not sure how comfortable I would be using it even if I needed to. My question is could I buy an 18" steel rim and toss on the best of my 4 Terra Grapplers and still fit it underneath my tuck in the stock spare location? If not could I buy the 17" equivalent (diameter wise) and fit it underneath? I just don't feel overly comfortable knowing that if I blow a tire I don't even have a back up. Would it cause any damage running a standard highway tire on one wheel and Trail Grapplers on the other 3 temporarily as long as they are the same diameter? The size would be the same but the weight would be significantly different. I would really rather not have to keep a spare in the bed of my truck and would really like to have an option other than tire plugs just in case of a major blowout.

Thank you for any help you may be able to offer.

(I tried searching and couldn't seem to find a definite answer either way)