Check for bearing issues like speedy said. If they appear worn or damaged, replace them. If you dn't change anything, some will reuse the crush sleeve, I prefer using shim packs and solid spacers in the 9". Either way, tighten the pinion nut until you have 15 inch/pounds or rotational torque for used bearings, 25 for new bearings, that's without the R/P installed. That should result in the correct torque on the pinion nut. If using a new crush sleeve, be prepared for a workout with the pumpkin in the car. I broke a quality 24' breaker bar and shattered two sockets, use hardened sockets, Craftsman frowns when you bring one back that's been abused like that, he he, I once used a hydraulic jack to crush the sleeve on a Suburban, very backyard and can be tough on your teeth. Once the sleeve has started to crush, go slowly to achieve the rotational torque. If you're not wanting to disassemble the differential, I'd torque the nut to a factory spec if you can find it, it will be high, probably in the area of 125 to 150 ft/lbs? Then check for excessive binding or resistance. Someone here may have a better procedure for that.
Replace the seal while the pinion yoke is out of course. Just pry out the old, I like to rim the new seal with sealer and tap it into place with a block of wood so as not to damage it, keep it square in the opening.
Once I got the sleeve to crush, I backed off the pinion nut, I locktite it and go for the final. I've never had one come loose and done many in my day.