have you checked the fliud in it yet?if not check it first.. if it's a manual transmission check the cables that run from the trannie to the shifter. make sure theres no play,and all the bolts and clips are in place on the shifting bracket, and if it's automatic then makesure there are no leaks which will cause the trannie to not run properly..torq-converter might be going out. I hope that answers your question. If the car is making grinding noises in the lower gears, you probably have a planetary gear going out, or it could be your clutch. The best thing to do is get it fixed immediatly or your whole tranny will fail. your car's "brain" tells it to only shift in 3rd when there is a problem with the transmission as a safety to keep from creating more damage to and already sick tranny. Check your transmission silonoid valve connection. should be located on the left of the transmission. one of the four prongs could be damaged
If your transmission is not shifting into third gear and overdrive it could be a parking pawl. This prevents the transmission from rotating.
Your transmission synchros might be going out. Also, shift slower and see if that helps. A Jeep isn't a racecar, so shifting slower is easier on the transmission. Changing transmission fluid to a different brand might also help your problem.
because it was made by General Motors. but also because your transmission is about to go. you should do a transmission flush. if that doesn't work you could always drive the truck through a mine field that might make it shift into 3rd or 4th more frequently..
there might be a short in the transmission wiring that might have happened when you swapped out the radio if it's stuck in third gear, that's probably the issue
In automatic transmissions the second to third syncronizers always seem to go bad first, the syncronizers are worn, bad or gone. This is common in these transmissions. Also you might check your trans fluid level, sometimes a low level will cause the transmission not to shift correctly.
Could be low fluid,a bad sensor or transmission going bad.
it maybe the alternator... warranty will cover you
The transmission shift solenoids are bad!!
On my 1990 Voyager, the transmission has a "safe," or "limp" mode. If something is wrong with the trans, it will not shift out of second gear. This is supposedly to allow you to drive to the repair shop at a reduced speed.
check fuses check for 12v supply at transmission-- wire @ 12 oclock on trans connector
Damaged/broken shift forks
Are you sure that it's third it's not shifting into, and not fourth? The transmission is designed to not permit shifts into fourth (overdrive) until the engine and transmission have warmed up a bit.