The rear plugs can be painful to get to. The right way to do it is to remove the intake manifold and number of brackets/nuts bolts, etc. to make enough room to get to the plugs. There is a reason why they use the long life plugs. Note that this is not unique to the Highlander, a number of other manufactures with V6 or V8 engines also have hard to reach rear plugs - some require removing the engine to get to the plugs!
2002 Corolla S, Silverstream, 1.8L 1ZZ-FE (VVT-i)
2003 Matrix XRS, Cosmic Blue, 1.8L 2ZZ-GE (VVTL-i) - RIP
2009 Matrix XRS, Blue Streak, 2.4L 2AZ-FE (VVT-i)