In reading the TSB's for 1969 it appears that the differential gear oil was changed. The plate type Sure-Grips were replaced in late 1968 with limited amount of spring loaded cones. 1969 were all spring loaded cones.
"LUBRICATION
Lubricant required for axles with the new cone type Sure-Grip differential just meet MS (Chrysler Material Standards) #4893. Chrysler Part No. 2933565 meets these requirements. MS-1569 Chrysler Part No. 2585318 must not be used in these axles.
MS-1569 Chrysler Part No. 2585318 (or its equivalent) must be used in plate-type Sure-Grip axles. These axles have identifying tags stating, Use Sure-Grip Lube or Use Limited Slip Lube Only.
Conventional Differentials - Part No. 2585317 or 2933565
Plate Type Sure-Grip - Part No. 2585318
Cone Grip Sure-Grip - Part No. 2933565
If you should install a 1969 model differential and carrier in a pre-1969 vehicle and you require using the original drive pinion drive flange, it will be necessary to install the 1969 drive pinion seal guard on the original flange. This new guard is not interchangeable with previous model carrier castings."
I think I retyped all this correct. I have no idea if this will help here but maybe the different oil has something to do with the problem with the Cone Grip Sure-Grip. This is mentioned in more than one TSB in 1969. The differential gurus might know what to make out of this. It appears that the Cone Grip Sure-Grip was an issue. Maybe that is why the housing was modified for 1970. The 1970 TSB has only 1 page on the outer seal replacement on the 8 3/4.
