I have 18" wheels and just had to mount the spare tire due to flat. The spare however, is 17" for an AWD vehicle. Is this safe to drive without destroying the driveline in the truck?

 

No. Limit the driving to get off the main road and MAYBE to the shop. That's it.

IndyD

 

Check the aspect ratios of the 2 tires. Regular tire is 265/60/18, which had an overall diameter (with GY Wranglers) of 30.4"; if the replacement is a 245/70/17, overall diameter is 30.6 inches and should not cause problems for temporary use, such as driving to the repair shop. I wouldn't go speeding around on the spare though. You can also check the manual for recommendations.