I think the climbing thing is the force basis of the vehicles problem. I tried a lot making the ural "work" properly at inclines and i still have that bad thing - you get force, but it doesnt have the needed speed-limits at certain gears.
The first gear is kinda too speedy, if the power should be great enough to climb stuff (mean the vehicle script thread one i use for the ural) and the higher gears tend to speed up to infinity if the highest RPMs aren't set to suppress the acceleration (still if the gas pedal would be intensity based and you would hold the max power RPM, you would exceed the vehicles speed limit, cause it wont be retarded as when you let RPM go to max where the power is reduced). Right now, i think the higher gears could be set good at it, but the first two - you can see, the acceleration, even whyte a 20 kmh max speed, is kinda odd i think.
... so its something in the way those forces are handled in time there. ( maybe some kind of acceleration constant added to each gear could fix that ) But it isnt bad for speedy cars, like Lamborghini's.