My 17 4runner had 1½" front and 1" rear spacers. It too could not get caster in spec and was "twitchy" at highway speeds.
Recently removed those spacers and installed Eibach Pro Truck 2.0 suspension front and rear. Also JBL upper control arms with 'exrta' caster. And a couple of spacers, lol. On the left side to compensate for the 4runner lean. ¼" left rear and ½" left front.
Ride height is the same as the spacers. Handling is much nicer as the caster is now in spec.
Added CBI aluminum skid plates at the same time.
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