Wow, sounds nice . . . got any pics? If I had the 20's, I would definitely realign the rears for as little negative camber as possible. I have the 19's and I am already down to -1.7 in the rear. I believe the 20's will show any mis-alignment better than the smaller sizes. I would surely get it checked if anything because those 20" tires are more expensive and you don't want to be going through them often.

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2010 Lexus RX350 still new
2003 BMW X5 3.0i w/sport package w/68K miles. (gone Thank God!)
2002 Dodge Ram 2500 4X4 cummins diesel w/198K miles, custom repo truck
2009 Scion tC w/46K miles
2007 Suzuki SV650 custom streetfighter
2000 Lexus GS400 w/165K miles