durango rear end options

hi, my uncle has a 98 durango with the v8(not sure the size), but when it is on the highway it looks like it needs another gear. when it's running 55-60 in 4th(overdrive), it's running 2,000 rpm, and it redlines around 4,500 rpm. most automatics i've seen, with v8s run alot lower rpms than that on the highway. this might be a little exagerrated, but it's pretty close from what i remember. does anyone know a way to lower the rpm's on the highway(it's killing him in gas)? i was curious if there were rear-end options available, because it's 0-60 is impressive for a suv, maybe that's a "sports" package. if there is a rear end option, does anyone have the ratios? or are dakota's compatible? ratios for that?

or if anyone has any feasible ways to get better gas milage on the highway

I thought it should run about 1750 in overdrive at 60 mph. Maybe the torque converter isn't locking up or something.

how do you check that? how to fix? replace? well i'll check his haynes manual, anymore help is welcome, thanks