a properly set up Dana 60 with any gear ratio and a spool should not turn hard under any street, straight line movement. Jack the car up and put it on jack stands and then try turning one wheel or the other to see how hard it turns forwards and backwards, if it is hard (you need to strain to turn the tire with both hands)then disconnect the driveshaft and try again. If it is easier with it disconnected look at the transmission
If it is still hard you need to find out why before you go any further
Good luck, let us know what you find
BTW, Mopar engineers use to recommend using one quart only of gear grease in both the Dana 60 and one in NP833 for drag racing only In fact we use to run one quart of ATF only in the NP833 for drag racing, that work real well