Ok my duster is a fresh build from front to back, and I have not driven it on the street yet. The rear has 3.23 gears and is suppose to be a suregrip, but when I put it in gear and give some gas on the slick garage floor only the passanger wheel spins, so Im thinking its a open end. But when I jack it up in neutral and spin the rear wheels they both spin in the same direction, and when it is in park the back wheels are locked tight and wont turn. Correct me if im wrong but these are signs of a sure grip right.

My axle guy "Shawn" told me that a loose, worn out Sure Grip will show the signs that you stated. The key is at which point the guts break free and allow one tire to spin while the other sits still.
The test is to have the 3rd member out for a test. Shawn sets the unit in a axle shaft bolted in a vise. He then inserts another axleshaft in the other end then puts a torque wrench on it. A new SG unit should hold 130 lbs of torque before breaking free. A decent used one should hold 90-100 lbs. I took an old Cone type in to have checked out. It failed at 30 lbs! In a straight line on dirt, it spun both tires. Turning right it would just spin the right tire!

You can do the same test without taking the rearend apart. Jack it up onto jack stands. Have a buddy hold 1 tire while you turn the other 1. If your buddy can hold it the unit needs rebuilt..

Sounds like a worn out sure grip unit. My Cuda did the exact same thing when I bought it.

If it's an original rear to your duster it's a cone type ... POs ... it's on it's last legs .

Jack one wheel off the ground , put it in neutral , choke the front wheels and try to turn the wheel that is off the ground .

How many parts referring to cone style POS last for over 40+ years? I have no issue with the cone style sure grips refurbish it and it will work for another 20-40 years