Smartest thing to do on any new purchase '64-69 GM with bolt in sealed bearing axles is get It up on a lift, or at least on jack stands. Pull the rear wheels/tires, the rear drums, and pull the axles. Get the darn things out and ck the archaic design sealed axle bearings. If the sides of the RW507 sealed bearing have a look of a green phenolic material, they are the original sealed bearings. Many didn't even make it 30,000 miless before seizing and chewing up an axle. They were designed for an age when abodys had skinny 7.75 tires, not 60 series tires, nor todays 17" or 18" hooptee specials.
Next, press those sealed axle bearings off, ck the axle for wear at the area the bearing was pressed on. If axles are chewed, count yourself very very lucky the axle has not pulled through the retainer and exited the car. That usually happens during low speed cornering, but have provided parts and differential work for owners that it happened getting onto a freeway on ramp at speed. At same time axles are out, pull the rear cover, ck the carrier and numbers stamped on the outer edge of the ring gear. single track carriers, they are windowed, or "open", easy to spot the side gears and spider gears and the single pin through the spider gears. Safe-T-Track rears, aka posi's, are easy to spot, they are usually more closed in design, ESP for the 2 pinion and 4 pinion cone style units. Can't figure it out, post a good picture with the cover off, should be able to identify.