[Jeff Chambers]

You could have any number of problems based on your description of the events. First off, with installing new gears AND diff you're probably wayyyyy off with bearing preloads.

For the pinion, you want to torque the pinion nut until you have about 18 in-lbs (note in-lbs, not ft-lbs) running torque on the bearings. The norm is 10-15 in-lbs for a used bearing, 20-25 in-lbs for a new bearing but right around 18 in-lbs is pretty safe for either. Too little preload and the pinion will move under both accel and decel. Too much preload and you'll burn the bearings up real quick.

For the carrier bearings, you want to slide the carrier in an begin adding shimming until you get to the point that the last shim installed has pretty heavy drag going in. This is the zero preload position. From there, take both shim packs out, measure them with a micrometer and add .006" shimming to each pack. This adds the proper preload to the carrier bearings. Arrows on the caps face outward.

To hold the pinion, you can build a pinion tool like I have, or you can try one of the following. 1) Install two bolts in the companion flange and use them to wedge a pry bar to counter the torquing force or 2) insert a hardened drift through one of the bolt holes so that it can be wedged under one of the stiffening webs on the carrier housing.

Don't check your pattern unless all bearing preloads are established; you'll get a false reading otherwise. Also make sure that you're setting your indicator perpendicular to the gear when checking backlash. I set street/strip backlash to 0.008-0.012".

Good gear setup is a game measured in thousandths of an inch. Patience and attention to detail are absolute keys.