sorry if this has been stated, but i've just skimmed the thread.
the reason it doesn't turn on gas is due, in part to the savs' 4 wheel drive system.
i'll explain- as there is no centre diff, as is common in all automotive 4wd systems, all torque from the motor is split equally between front and rear diffs, making it understeer, as too much grunt is going to the front end.
i had the same issue, and i found running a really thick grease (car cv grease) in the rear diff and a light oil in the front has helped no end, in my oppinion STAY AWAY FROM DIFF LOCKS, especailly if you're going round corners- it will break the casings/driveshafts/other bits!!
the idea of a diff locker is for hill climbs. going to the main properties of a diff, it is there to allow the vehicle to turn corners, as round a corner the inside wheel travells a shorter distance than the outer, so the diff allows cornering to take place without stressing components, now this is a good thing, but it does mean that if one wheel on an axle (or a different axle with a centre diff-not a torsen, these do not have the same issue) looses traction, it's like electricity, it will take the line of least resistance, thats why when the sav is on its' side, it will spin the wheels in the air.
so there we have it, a load of rambleing!
also, on another note, it might be worth looking at a mid tank mod, because it puts more weight on the front end (until the tank gets low!!) and it looks cool to boot!!