The rims sit further inward so they will rub your front leaf springs when turning.
Also, by moving the wheels further inward your reduce the stability of the truck. M0oving tires outward increases stability (bigger footprint).
Stay away from spacers ESPECIALLY if you are towing. In stock form, all of the weight of the rims is placed on the hubs, NOT the lugs. If you put spacers you put the load on the lugs and they will fail.
You can get 18" rims from the 2003/2004 Harley Davidson or King Ranch editions. If you want larger you will have to go aftermarket.
Increased bearing wear is a stretch. I can see if the wheels have a huge positive offset and stick out way past the fender wells. But the OEM 20's (et al) will still be centered underneath the hub with the spacers and will exert minimal additional leverage on the bearings.
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