If your car has a manual transmission, and it over rev's in every other gear as well, than no doubt it is your clutch. Other wise it is your transmission.
When shifting a manual (or standard) transmission on a vehicle, if you hear excessive knocking noises immediately after shifting, you are probably:1) Shifting into the next highest gear too soon, or2) Shifting into the wrong gear - for example, going from 2nd to 4th gear, or3) Not running the engine fast enough after shifting to the next highest gear, or4) Your engine is out of tune if you know you're doing steps 1,2 and 3 correctly
A lot of these vans had software upgrades for trans shifting issues. The dealer can "flash" new software into the computer.
N/M
Your car if its a stick is not pulling out of gear all the way and your moter overrevs
Synchronizer maybe. not sure if that's how you spell it. I grind a little downshifting into second but not when shifting into any other gears. A mechanic buddy told me that's probably what it was.
The first cause of the jump out from one to other gear is the loose shifter fork, which connects the shifting linkage and gears of the transmission. The second cause is loose shifting linkage itself, which connects the gear shifting knob and shifting fork. The shifting linkage and shifting fork become loose due to the rapid changing of gears from one gear to other at high speed of vehicle. The third cause is the misalignment of shifting linkage to the shifting fork.
If it is like most Chrysler Autos, you need to put your foot on the brake pedal before shifting into gear.
The engine is turning more revolutions at the lower gear. Once you shift into a higher gear the rpms drop.
Your sun shell is broke
The car isn't necessarily shifting into neutral its just staying in a lower gear and not shifting to the next gear, check your transmission fluid.
If your transmission is not shifting into third gear and overdrive it could be a parking pawl. This prevents the transmission from rotating.
The biggest problems of why a jeep wont go into third gear is either that the shifting fork for third gear is worn or bent or the sycronizer or gear itself is worn out and needs to be replaced. Once you take the top of the transmission off where the shifter is located you can see the whole transmission gears and inspect the shifting forks. A general rule of thumb is to make sure the transmission is in second gear before taking the top off. This will make it easy to take off and rerattach the top of the transmission cover and align the shifting forks.