I would tend to lean more toward the RPO code in the glove box being correct. If the owners manual is SUGGESTING at gear ratio, that is probably the standard gear ratio, but there are several others available. In a 1500 I much doubt its the full float rear. This 14 bolt that I got recently is a semi float and has 4.10's. You may also look for a G80 RPO code in the glove box too. If it has the G80, and is still original, then its locking differential. Not one of the better ones, It will be the Eaton Gov Lock.