Ive had good luck with Superlift. As far as "sagging" in the rear, 99% of lift companies use blocks, so if anything is going to sag, it would be your stock springs. If you do a ton of towing heavy loads, then your more likely to have airbags anyway. Make sure though, if you buy the kit, you obtain the carrier bearing bracket that lowers it, otherwise you will likely get a vibration. If your buddy doesn't have it, its like 70 bucks. Its also a good idea to spend the extra money and get bilsteins, my truck rides like a caddy. Good luck.