transmission locks up

1998, runs fine when cold, but when it warms up, the tranmission will lock up going down the road(scary to say the least). Anyone have any ideas on this? I hate to think I have to buy a new transmisson.

first thing I would do is change the fluid and filter before I drove it anymore.

locks up?? you saying it wont shift to other gears?

The transmission was slipping a little so I did change the fluid and filter, it ran fine for about a week.. When I say locks up I mean you will be going down the road and the wheels will lock up, Skidding hopping, you don't really have time to try other gears, You are too busy trying not to wreck..lol. My wife was driving so I can only imagine how it shook her up. By the time I got to her(about an hour) I could limp it home in second gear. First and second works, but when you put it in drive, it's like you stomp on the brakes.

but when it cools down every gear works, and works like it should

One more note; the transmission temp light was not on prior to locking up; but once it locked up it came on

Is it 4wd? Don't discount the transfer case if so. It is more likely for the transfer case to have a messed up shifter letting it hit two gears at once. The two shifters forks in it are one plastic, one aluminum. Otherwise you may have a tooth from a gear floating around and getting caught between two teeth. That is a weird symptom, don't really know right off. May be a take it apart to look problem.... If it was in the valve body it would probably be predictable/repeatable....