I looked at the diff and it is what is considered to be a GM Corporate 14 bolt (12-1/4" ring gear) rear end. There is no mention made of needing shims for any reason. Here are the instructions for replacing the drive pinion oil seal:
1: Disconnect the propeller shaft (dah!)
2: Scribe a line down the pinion stem, pinion nut and companion flange.
3: Use J-8614-11 to remove the pinion nut and the companion flange.
4: Remove the bolts retaining the oil seal retainer to the carrier, and remove the retainer. See figure 77
5: Pry the oil seal from the bore, using care not to damage the machine surfaces. Thoroughly clean all foreign material from the contact area.
6: Lubricate the cavity between the seal lips with a high melting point bearing lubricant.
7: Install a new pinion oil seal into the bore, using J-22281. Be sure seal bottoms against shoulder in bore.
8: Install the bearing retainer to the carrier.
9: Reinstall the companion flange, pinion nut and propeller shaft.
You might check your pinion shaft to see if it is scored or worn. If it is you can probably get a speedi-sleeve to repair it with.
Hope this helps.
Air forums # 1674
1974 20' Argosy Motor Home
1975 24' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels & windows left to sell)