Just wanted to run this pass you guys,,I just have had my first experence with a bad sure grip,I have had plenty of well worn ones in the past but none that did this. I installed a pumpkin(8 3/4 742 case) that was sitting around in my 64 last week.Everything looked good and it was a ratio that I wanted so in it went,backed the car out of the garage, level ground,put in park on a slight grade and the car started rolling down the drive.Hit the brakes,put it in drive and the driveshaft spins but the car don't move. Same in reverse now. I've heard this happening from others but ?Will the clutch pack in the diff be worn that much to allow it to do this ? It's a first for me... gonna tear into it this week to see what's up.

The axle splines, pinion or side gears, inside the posi carrier have to be broken or completly "stripped" for that to happen.
Is this a "clutch" or "cone" posi unit?

It is not uncommon for cone (Auburn Gear) posi unit to wear to the point where the gears inside the unit move away from each other and then fail.
It is less common for the clutch posi units to strip-out the gears inside. Those tend to do axle spline / side gear spline failures.
Make sure the axles are fully installed, completly seated into the center-section. Did the axle shaft end play, come-in with-out lot's of re-adjustment?
DD

Thanks Doc, V ramp clutch type. Axle splines good. Green bearings no adjustment. I'm thinking the clutch pac splines maybe the problem. Will look when I get it open. Just was wondering if anybody else had something like this happen.

Good info...
If the unit has the "double" set of axle splines (older style) then my guess is that the axle shaft(s) do not go in deep enough to "grab" both spline sets, on each side of the carrier.
Fact is, the later versions redesigned this area and has a single splined area per side, that avoids this issue.
DD

Doc you nailed it in your last post, It is the older double spline.
I just dumped a 2.76 open unit in it for now.
I'm going to order new clutch kit and go through the suregrip anyway. When I pull it apart I'll measure the axles and see for sure. Mix and match with used parts,,, live and learn.

There is an up-grade kit available to change the unit to the later "single spline" set-up.
Give Dr. Diff (Cas) a call, he has them.
DD

Plan on it.Also going to get a spool from him.for my 4.30's and put them back in the Dart.