GM-GMC: problem accel during overdrive, chevrolet silverado z71, rear axle ratio

Question
I have a 99 chevrolet silverado z71 with the 325 5.3 ltre it is a single cab sport side.  My dad has a 2000 chevy silverado reg cab long bed z71.  I have noticed that while in over drive running at low rpms his truck will accel much easier than mine.  For ex. at 50 mph while keeping rpms low around 1500 rmps the 2000 silverado will reach 55-60 much easier than the 99 which is a lighter vehicle.  It will also hold speed on hills easier without having to raise rpms.  Is this caused by maybe different gears in the rear ends, less power or what?  If i didnt specify problem enough let me know, thanks drew

Answer
Drew,

It is very likely a rear axle ratio difference.

In the glove box, or out on the fenders under the hood, you should find a list of Regular Production Options (RPOs).  These are codes for the equipment put on each truck at the factory.  Find the ones for the rear axles and then call your local dealer.  The dealer can tell you what the ratios are by the RPOs.