It is a well proven and reliable system. Unrefined by today'standards and delivers a rough ride.
Nissan basically use a D21 platform for the D22 therfore the torsion bars. Honestly Nissan have no interest in fitting coils to the D22 as the D40 has coils, from a marketing perspective keeping the D22 and D40, Chalk and Cheese and the appropriate variance in retail value is a good idea. they make a truck for more buyers this way.
Regards
Greg