Prius Shim Adjustment Procedure

Introduction

To align the rear wheels, you will need multiple trips to an alignment shop to have the changes checked. But an alignment shop will typically charge $60+ for each report and you will need 3-5 visits, potentially $180-300. The right answer is to buy a "lifetime" alignment, typically $140. Not to worry, you can use it every time the tires are rotated to make sure things are still OK.

You will also need a set of metric sockets; an accurate torque wrench; and a floor stand jack ... NEVER WORK UNDER THE LIFT JACK. These can be found at Harbor Freight or other discount tool places. Just remember, you health and life is at risk so spend the money for the right tools.

Initial Survey

Get an initial alignment reading and look at the rear wheel values. The Prius has a 'floating' rear axle that is supposed to self-adjust. Identify the wheel with the worse toe. This is what we'll fix first:

Jack up and remove the tire - make sure the car is held up by a jack stand, not the lift jack. You are going to apply significant torque later on and you don't want it to roll off the flimsy lift jack.
Remove the rear drum - just rock it a bit and it slides off ... assuming you didn't set the parking brake. If you set the parking brake, release it and get the rear drum off.
Loosen but do not remove the four hub bolts - located on the reverse side, use the right hand rule (curl your right hand fingers and stick out the thumb; the fingers are rotation direction and the thumb is bolt direction.) You want enough of a gap that the brake and hub assembly has a gap with the axle.
To fix excess positive toe, put two of the smallest shims on the front bolts. If you have excessive negative toe, put two of the smallest shims on the rear bolts.

This is the backside of the driver side, wheel hub and you can see the anti-lock brake sensor socket. To remove the plug, you'll need a brush to clean the road grime off of the top the plug and a small, flat screw-driver to pry up the lock. But the shims are a little too long to fully seat.
Cutting off 1/4" will shorten the legs so the shims will fit closer to the bolt.

Shortening the metal tab legs provides more area to carry the loads. Tighten the bolts and make sure the shims are seated.
Tighten all four bolts to 38 ft-lbs. (NHW11). If you have an NHW20, you'll need to check the Haynes or other manual to make sure the bolts are torqued correctly.

You can just make out the end of the tabs sticking out between the hub and axle. The top shim will stay in place with gravity but the bottom one takes fingers holding in while tightening the lower bolt until it clamps the shim.
Take the car for an alignment check - if the correction was not enough, repeat the installation using the next larger shim.

DO NOT STACK SHIMS!
ALWAYS USE IN SAME-SIZE, SHIM PAIRS!
TOE USES ONLY THE FRONT OR REAR BOLTS!

Try not to repeat my mistake of correcting positive toe by putting a large, 1/32 shim on the rear bolts:

I started out, the upper left, with 0.30 degrees of positive toe. I put a 1/32 shim on the wrong bolts, the rear ones, and drove the toe to 0.90 degrees! But when I put the 1/64 shim on the front bolts, taking off the 1/32 shims, it came in at a very nice, under 0.10 degrees.

You'll notice the camber changed as the toe was corrected. This happens because the bolts are slightly rotated so there is a toe-camber relationship. Of the two, getting the toe right is the most important since this causes a scrubbing action on the tires. Once the toe is corrected, adjust the camber on the opposite wheel:

Take off the opposite wheel and loosen the four bolts.
Assuming you have too much negative camber, put in a pair of small shims on the top bolts.
Tighten the bolts and torque to 38 ft-lbs. (NHW11) or the equivalent torque for NHW20 bolts.
Test the camber.

If the camber isn't enough, repeat the process and use a larger pair of shims. Always use a pair of identical sized shims so the opposite bolts will be torqued correctly. If you have mismatched shims, you will have a rocking motion and that can over-stress and fail a bolt.

Preventing Rust in Gap

The pairs of shims will leave a small gap between the hub and the axle with the largest gap on the side with the shims on the bolts. If you live in an area with a lot of salt in the winter or near salt water, you may want to put a layer of paint over the gap or at the gap. This will minimize exposure to oxygen and water to slow formation of rust. The bolts are treated and should be OK but the base of the hub is bare metal and the axle plate is painted. In practice, after two years, there was no significant debris in the gap and no significant rust. But I live in an area that seldom uses salt in the winter.

There is a more sophisticated alignment system, the "EZ SHIM" but early tests have been disappointing. I don't trust their instructions and my updated procedure is still a 'work in progress.' In theory, the "EZ SHIM" handles all four bolt offsets and eliminates the gap left by the shims.