Neil NJ 99-FR-FT4WD-5.9
franklinnl@nswccd.navy.mil
I'd not go by the advise of the tire guys if they are saying the tire was not meant for the rim as it is the stock size. I believe you heve their recommended tire size wrong. 285/65/17 would be huge. 31.6 in diameter.
As far as getting 20" rims, the rim diameter does not matter. You just need to get the correct size tire for your application on those 20s.