Replacing single tire OK?
 

I have a 98 F150 XLT 4X4 with the off road package and about 33,000 miles on it. One of the rear tires was punctured and had to be replaced. I put the new tire and the spare (like new) on the back and left the existing tires on the front. Will the slight difference in tire size from front to rear cause any problems? The tire dealer wanted me to buy new ones all around and I would prefer not to unless it will cause other problems.

The tires are all the same size except for wear . The tire dealer had his micrometer out and said something about damage to the transfer case if the tires were more than a few hundredths off from each other. I thought this could only matter when in four wheel drive and that you should only use four wheel when on a surface where the tires have some slippage ( sand, dirt, ice, etc.) which will relieve any binding when turning, etc. I just wanted to make sure I wasn't missing something.

how much difference
 

How much size difference are you talking about? If all your tires are the same listed size, I wouldn't think it would make that much difference having 33,000 mile tires on the front and newer tires on the back. I was always taught that you buy new tires either 2 at a time or 4 at a time when replacing old/worn out tires and that you put the newer tires on the front. I see a lot of cars and trucks with either the front or the back tires of differing age/mileage but the same listed size. If it was right versus left, I think you might have to be concerned about handling, etc. I would probably have replace both rear tires to have them the same age and left the spare where it was. Just my 2 cents.

Sounds like you did the right thing to me as long as your spare is in good shape. Its far more important to have equal size tires left to right that front to rear (within reason of course). Only when you're in 4 wheel drive will it have any effect, and even that will be insignificant given the slight difference you're talking about.

Your fine as long as the tires are the same size. If you have limited slip rearend you do not want to mix sizes as it causes excessive wear.