I need to find out what transmission is in a 1999 gmc sierra 1500 pick up 4x4 extended cab.

It is one of the following: they were available in a 1999 GMC Sierra SLE 1500 4X4 Extended Cab~~~~ 4-speed 4L60E automatic 4-speed 4L65E automatic 4-speed 4L80E automatic 5-speed manual......now to figure out which one you have, go to: Official GM car and truck VIN decoder cards: http://service.gm.com/dealerworld/vincards/

Okay you have to go to where?

Got there but don't see anything about transmissions

OK. You need to have your VIN#, paste that in, find your year/make, click on it. Hold on I will show you another one

Go to: http://www.gmtruckclub.com/forum/showthread.php/30241-GM-car-and-truck-VIN-decoder-cards

Yeah I got the VIN# and checked it against the VIN-decoder-card but still don't see anything about transmissions.

OK. I thought there was a way to find out from you VIN# on those sites, there may not be, you have to navigate through all the codes and such. So Ok that didn't work. Next suggestion is crawl under the truck and look for metal tags or plates on the transmission, and /or numbers and letters stamped on the case.. it may not say 4L65E, or whatever, it may be in code also, look for any markings. But you don't say if it's an automatic, it can only be one of three, listed above all 4 speed automatic, unless you do have a manual, then there was only one offered that year for that truck. Good luck. if the transmission is bad and you KNOW that for a fact, just go ahead and pull it out if this is a DIY project anyway, and the serial number and which one it is may be stamped on TOP of the case. PM me your VIN#. I think I will be able to tell you what you have.