So, for starters, where do I post questions I might have about a '99 Nissan Hardboy?

Since I don't know that yet, the person who owned my truck before messed up the suspension pretty bad, and I aim to fix it. So who makes sport suspensions for the '99 extended cab model?

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Nissan stopped making the hardbody in '97. In '98 they started making the Frontier. From the looks of your truck you either have a '94 or '95 model. To fix the problem with the suspension just buy new leaf springs.