8.75 SureGrip , Is this Normal?

I think you got the right idea.
Maybe you left a rag in there and it got jammed in the ring gear. Or something like that.
Once the pinion won't turn, then turning the axle one way on one side, HAS to turn the other one in the opposite direction. It HAS to. But in a cone-type SureGrip, it should take about 130ftlbs of force to do this.
And like you say, there are two parts on each side that the axle has to engage; the outer one is the sidegear, and the inner is the actual clutch, on each side. They (all 4) are normally aligned by the assembler.
The trac-lock can be set up a little looser, cuz it operates a wee bit differently. The cross pins are driven up ramps by the driveshaft (OK by engine torque), which increases the clamping force. I still set them up about the same.
But the pinion should rotate fairly freely,in any case.