In the under $500 price range, you are looking at lunchboxes and limiteds. Lunchboxes generally are not recommended for rear axles, and of the limiteds, the truetrac is the best option.

A front d30 TT should be around $375, and a rear d44 TT is around $460. Autolockers start around $550 or so, and selectables start around $850, plus any extras like air sources or labor to run cables, etc.