These coils will give you about 3" of lift (also available from Rocky Road Outfitters).
http://www.independent4x.com/Isuzu-O...gs-_p_132.html
There have also been some members who have used coil spacers rather than replacing the springs themselves ... but your springs are 15 years old so it would probably be a better option to replace/upgrade (just my $0.02)