Bren--- According to both the General Tire & Tire Rack websites your MAX PSI is 80, not 70. Thus, let's properly identify the exact tire in question.
You have the Grabber AT2 in LT245/75R16-120/116S (30.5" O.D./44.4 lbs), Item #568408000 which is a E-Load Range/10-Ply tire with a MAX PSI rating of 80 PSI (per tire sidewall panel).
At your current PSI setting of 58, your AT2's are way too underinflated and it is helping contribute to the bouncy feel you are experiencing. Further, your MPG and tire wear is not the best it could be with the increased drag.
You will get the absolute best MPG by running the tires at the MAX PSI of 80, and your tires will turn easier when parking and turning street corners. The Grabber AT2 is a very "meaty" tire and it will cause a lot of drag and rolling resistance if not inflated to the higher end spectrum of its MAX PSI rating.
My personal recommendation is to inflate your AT2's to the MAX PSI of 80 and to adjust your Rancho shocks to a more softer setting as you described in your initial post. If after riding at the 80 PSI setting for a while; you deem the ride too harsh for your personal tastes, then you can gradually reduce the PSI setting in incremental steps more to your liking. However, I would not go any lower than -10 PSI of the Max Rating (in your case, no lower than 70 PSI of the 80 PSI allowed). Give your AT2's a thorough test at the 80 PSI setting, as I believe you will like the firmer and more responsive ride while attaining the very best MPG.
![]()