when I'm driving on the highway in 5th gear, is it normal for the steering wheel to shake when my speed drops.

 

No.

 

What do you mean when your speed drops...
do you mean when you apply the brakes at high speeds?
or do you mean when you lift off the gas and coast to slower speeds?


things to consider:

- when you got your tires balanced, did the shop balance them on the outside AS WELL as the inside. Some shops only balance wheels on the inside, its better for both sides. The advantage is more stability over high speeds.
- if the wheel shakes while you apply the brakes... check your wear and tear on your pads and rotors.
- maybe the cable in your steering wheel has become loose.

 

if it shakes bad when you apply the brake , check your tension rods also as said before get tire balanced

 

True...your best bet are tires and rotors.....