It may be that somebody set the turning torque all wrong, and now your diff is acting like a spool.
AND the backlash on the C&P is supposed to be less than .010 at the tightest point, and should not vary by much all the way around. Usually .007 gets the pattern darn close. The pinion will never clunk if set up right! (see note-1), AND Ima thinking something is bent, or the one-way clutch in the back of the trans is not working right, or the U-joints are seized.
I would start with dropping the driveshaft, and testing the sprag and checking the U-joints. While the shaft is off I would retest the turning force at the axle ends, in the same manner as you previously tested. (see note-3)
Even if the SureGrip is bound up solid;...With the wheels and drums off, and the recommended small axle-bearing end play; greater than zero/less than .007;......you should be able to rotate the pinion by hand, with very very little effort.
If you cannot, then remove one axle shaft (See note-2), and try again. If it is still dragging, remove the other one and try again. If it is still tight! remove the pumpkin and send it back.
But if it loosened up with either axle out, or both, then something is bent, or the bearings are NOT all on the same axis; that is to say; the axle bearings and the case bearings. This points to a bent banjo housing.
If when you remove either axle, it doesn't practically jump out of the housing, this also points to a bent housing.
Note-1
The only things inside the cone-type third member that can clunk, are the spider gears climbing up and down on the cross-pin or, the crosspin slamming around in the saddles or, the crosspin is too long and is hitting the pinion; NONE of which should be happening! in a just rebuilt unit.
Note-2
I say remove the axle, but mean just far enough to pull the splined portion out of the sidegears. The trouble with only going this far, is that the outer end of the axle will now be resting on the seal. That could be bad. So I say take it out; but when you do, it will make a mess so have a bucket or tub available to capture the drippage. Alternatively, you can support the axle flange in some way to prevent the weight from going onto the seal.
Note-3
BTW the installed driveshaft only likes to turn one way, due to the sprag in the back of the trans. When you spin it in reverse direction, the sprag locks and forces some of the transmission guts to turn, and one of them drags on the L/R band, which creates the major part of the resistance that you will feel. In the opposite and normal direction, the sprag slips and the only moving parts are the planetaries which you can barely feel.
best of luck to you.
