Hi Banjoman
I've driven from Vilnius to Klaipeda many times, as I've previously lived in Lithuania.
There shouldn't be any problems, but of course watch out for speeding drivers. Driving ethics in Lithuania are somewhat questionable.
The other thing is the stretch between Vilnius and Kaunas which goes through many little towns (Ziezmariai, Vievis and so on) and obviously the driving speed limit drops quite regularly - watch out for pedestrians as they tend not to stick to the marked crossings.
The part between Kaunas and Klaipeda is very straight forward (literally!).
You will also see a lot of students trying to hitch-hike from Vilnius to Kaunas and further on to Klaipeda. It is up to you if you want to share a journey with the natives!
If you are considering renting a car while in Lithuania, try this car hire comparison site
http://www.car-hire-centre.co.uk/rd-lithuania/
Best of luck!
Gyvybe