2004 Lexus ES 330 Replacing Struts

I have a 2004 Lexus ES330 with 93500 miles. I have been told by a tire changer at Wal-Mart that my rear tires show "strut bounce", and need to be replaced.

Since replacing all four struts can cost upwards of $250 per wheel, I don't want to un-necessarily replace them on the word of a young boy who might be trying to generate business.

My question is, "how long should I expect struts to last on this car?" I do composite driving. When I purchased the car, I was living in rural Mississippi, and spent most of my driving time on trips over 25 miles. A lot of highway driving of 150+ mile, one way.

I now live in a city of approximately 80,000 and make a lot of short trips 25 miles or less.

Are the struts getting near the end of their life? If they are, what can I expect if I just get some new tires and do not replace the struts?

Thanks,
Ed Wood