The rods are cheap enough at the hardware store, buy enough to run between the longest section between rodding points, start at the closest end to the house and work your way towards the septic tank. Afterwards flush water through and see if you can see any water collecting, indicating a negative slope in the pipes, and a pretty fair indication of future troubles or whether it's just a once-off occurrence.
It's an appalling job, but I'd reckon it's better to do it yourself and know that you've done the best you can, rather than paying someone to do the bare minimum. Not to be cynical or anything, of course...