2005 Pontiac shaking when I slow down.

We cannot diagnose your problem online. I would try a better tire balance. Look for shops that can do a Road Force tire balance. Have them check the wheels for runout and the wheel bearings for play.

You should also have the front suspension checked for damage and wear. Your car is old enough that it is likely time for new bushings, ball joints etc.

Sounds like tires have been ruled out. I've had badly worn out axle shafts do that and yes, at higher speeds they tend to smooth out. Worn out A-arm bushings can also do that although they are generally not a problem in these cars. As was mentioned above, I'd have a full front end inspection done by a competent front end shop to see if other components are due for replacement as well.

front end alignment

I'd start with alignment

well after balancing