How do tree roots get into PCV sewer lines?  I've had this problem in
the last two years and don't want to have to dig up the whole sewer
line and replace it.  Is there any way to stop the roots from getting
into the pipes?

1.) are you sure the part of the line they are getting into is
pvc(plastic) pipe?  often your trunk line will run into a main line
that may or may not be plastic(could be ceramic).

1a) if you're sewer line has tree roots growing through it then the
odds are it's colapsed in places, look for a low spot in your yard
heading out to the main sewer line.

2.) In terms of stopping the root growth, if indeed that is what's
happening, short of killing said tree, you're pretty much sunk.

3.) If you don't have a clean out installed in the front of your
house, you can get one installed for probably about what it would cost
for a new trunk line.... about 1-3 K.

4.) If you're main line is clogged, check your homeowners policy, it
may be covered.  If they have to dig up part of the street(happened to
me in Los Angeles), you're looking at 8-10 grand.

Chugs.