Best to go to a GM dealer with your vehicle VIN (would be on your
registration), then have them give you a printout of your "Regular
Production Option Codes" (RPO codes.) Then from that they can tell you
specifically what you have.
And if you have the printout, you will be able to look these up on the
internet in the future.
Open the glove compartment and you will find the RPO sheet with a
listing of codes that show the options, gear ratios and other info about
the truck.
The trans code to look for are
M30 = 4L60E trans
MT1 = 4L80E trans
And if you see the code G80 you also have the GM version of a locking
rear axle.
--
Steve W.