I'm really familiar with the ultrasonics and I'd like to suggest you try IR sensors instead.
I can recommend these:
Robodyssey.com IR sensors page They sell two different Sharp sensors that come in two different ranges. At the bottom of the page they have a nice mounting bracket that I've used with great success. Cables, sensor pair, mounting for under $50
The ultrasonics are sensitive to the materials around them, sounds don't bounce exactly like you think, carpet sucks them up and non-flat surfaces (like a ball) don't reflect the way you want. IR with the much shorter wavelenght has better bounce.
The Sharp IRs are my "go to" sensors for finding things 5" to 5' away.