The yellow stuff from old gasoline is varnish ( as in the stuff that is applied to wood ) Sitting 2 years isn't so bad, at least most of the fuel will still be liquid.

Carb cleaner will remove it, as will denatured alcohol. I'm not really sure what diesel will do. In any event sitting dormant with diesel fuel in it isn't likely to do much.

At least on later cars, the strainer in the tank has a coarse screen for diesel and fine for gas so changing it to coarse and swabbing the tank at the same time would be helpful.

Given the car has independent rear suspension, rear ride height won't affect driveshaft angularity. Front ride height will never change drive shaft angle regardless of rear suspension type.