Why is my transmission popping out of gear?

I've recently been having problems with my jeep popping out of 1st and 3rd gear. I know that this is a syncro problem or something inside the transmission. But yesterday I was driving to work and my jeep popped out of gear but it was different since the shifter didn't actually pop out. I'm not sure if it's a problem inside the transfer case since as soon as it pops out now the speedometer speeds way up until i let off the gas and let it pop back in. I've already bought a rebuilt transmission to put in but am wondering if I'll need to also replace the transfer case. I know this is confusing but any help is appreciated!|||I THINK IT IS JUST YOUR SYNCHRONIZERS THEY ARE WHAT ALLOWS IT TO GO BACK AND FORTH BETWEEN GEARS AND IT IS COMMON FOR THEM TO GO OUT !!! I WOULD TRY THAT BEFORE YOU SPEND A TON OF MONEY ON A TRANSFER CASE!!! I HOPE THIS HELPS YOU!!|||I would say most likely yes defintly replace case, or at least see if something has been striped or may be worn down.|||Sounds like it is stripped. I remember it happened to my uncle years ago. You need to get it fixed.