How can I raise my 1990 2WD truck to the height of the 4WD ?

not sure you can,check with auto store to see if they sell a lift kit for your truck.

These use a leaf spring in the rear and coil springs in the front. You could install helper springs for the rear, or blocks, that will raise your vehicle 2-3". These are available from places like JC Whitney. If you need a manual for this truck, let me know. I have the original factory shop manual for 1991 Mazda B-Series, which is hard to come by.

go and buy the entire suspension for the 4WD model and swap over the parts.