8.75 SureGrip , Is this Normal?

Bad plan man!
Something is physically preventing the pinion from turning,and I believe it is one of the cones or axles.Consequently when you force the pinion to turn, you are forcing both clutches or cones to slip and the side with the least friction turns.The resistance you are feeling may be the clutches. It may also be in the pinion but I think IIRC that you said with no axles in, the pinion turns normally?
Something is preventing at least one axle from turning freely. If you assemble it this way and drive it this way, I'm pretty sure it will burn up.
Again, this is not normal.
Also the friction modifier goes in with all/any EP oil, except one that is marked specifically for SureGrip(LSD) units,which should already have the modifier in it;read the fine print. When in doubt slam it in there.The SureGrip is just Chrysler's name for a type of LSD, of which there are maybe 4 or 5 types, not all of which are made for Chrysler products