Your 175/70R13s (should be what's on there) are almost identical in diameter to the infamous 165/65/R14 Potenza RE92.
http://www.tirerack.com/tires/tires....t&autoModClar=
Swapping to those would make about 10x the difference all the bearing changes you could manage combined. The stock size should weigh about 17 lbs, the Potenzas weigh around 13, but your larger wheel diameter will offset this somewhat.
http://ecomodder.com/forum/showthrea...e92-19126.html
And I'll second the turbo addition... probably good for something like +10% on a diesel. Why throw all that free energy out the exhaust.
Everything else seems like it's already been said... the belly pan (yes!) and kammback cover for the bed will help a chunk. I love those rabbit diesels, but they have the aerodynamics of a brick. I'm actually surprised it's as low as quoted earlier in the thread, I would have guessed north of 0.5.
Sam