If the truck is usually 1500 rpm @ 70 mph, and it jumps to 2300, it will be staying in 3rd gear (not grabbing 4th/Over Drive). Torque Converter lock is only good for around 400-500 rpm. 1500 is very low though, what size tires do you have? Does your 1st gear work properly?sounds like TCC. Is the truck cold when it does this? the TCC won't lock till the truck gets up to operating temps. Next time when you feel the higher RPM drop it in 3rd (direct drive) and see it the rpms jump up, if not then OD is not engaging, if the RPMS do jump up then it is TCC related. Not hard to diagnose. Also you can lightly press on the brake with your left foot as you drive and see if the RPMS increase when and if the TCC disengages.