What's wrong with my transmission?

I have a 93 jeep grand cherokee laredo straight. 6 automatic and having a problem with the transmission. When I start it up and drive it, it works for a couple of minutes. It seems once I get it warmed up or something all of a sudden the gears shift and it weirds out trying to decide which gear to stay in. Then I can only drive in 2nd or drive It doesn't shift into overdrive after it warms up. AND I have to manually put it into 2nd. It's becoming quite a nuisance I fear I might have to replace another idler pulley from the belt spinning faster than it needs to and I have to constantly switch it in and out of second and drive.. My question is, what's the problem and what can I do to fix it?