Car diffs tend to be quite a reduction, from around 3 to 6 to 1 on average, however with one side held the other output will run at twice the 'normal' speed, so a 4:1 will act like a 2:1, run the diff backwards (i.e. put the power in through the halfshaft and out the prop flange) and it should work as a 1:2.... I think I am right in this, someone (I hope) will confirm in a minute
I've no guarantees on the reliability of the above, as the diff is effectively working in 'coast' mode the whole time, however given the small amount of power/torque you are looking at using it may be OK for a long while.
As for small cars, think Austin A35, Morris Minor, Marina, Chevette, Suzuki SJ's, in fact small 4x4's could be a very good/cheap source. There are also other options, self-contained diffs, like 4 wheel drive road cars, and some rear drive cars like the Rover P6, Jags, Impreza, Audi Quattro's of all flavours (80,90, 100 etc)
Any clues as to what the project is?