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blykins: be careful that you are talking the right torque - 15-20 in-lb is the "rolling torque" of the pinion shaft. The nut is actually tightened pretty tight - somewhere around 175 ft-lb.

Having it get tight may not be all that bad - the bearings are set in preload and will be somewhat tight to rotate - that's the 15-20 in-lb. If it is too tight, there is a problem.

The crush collar is what provides the preload to the system. With the solid spacer, the bearings should be measured and the spacer made the correct length to provide the preload. This spacer is then a matched part of that bearing assembly and if the bearings are replaced, the collar will have to be either replaced or remachined to provide the correct preload.

Get out a torque wrench (in-lb) and see how much it takes to rotate the pinion. Either that, or a fish scale at one of the yoke holes. Measure the radius of the hole from the shaft centerline and multiply that times the scale reading as you rotate the pinion. If you're way more than 20 in-lb, you'll have to investigate and probably replace the crush collar.

If you're close on the preload, driving may help because the gear forces "crush" the collar just a little more and end up with a little less preload. The danger is that if you are too tight, the bearings will generate a lot of heat and could burn up. If you are going to do this method, I'd suggest just doing a trip around the block, but try to put as much load on the back end as possible (i.e. several fast starts, downshifts). Get out and check the heat in the 3rd member, then pull off the driveshaft and recheck rolling torque and that the yoke nut is tight.

Hope this helps.