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If your transmission is not shifting into third gear and overdrive it could be a parking pawl. This prevents the transmission from rotating.
Be sure the trans is full with the proper lubricant.
3rd gear is bad,must put new gear in or replace transmission.
If it is automatic and it grinds, that means you must come to a complete stop before shifting into reverse, or your transmission is no good
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 vibrates briefly only while shifting, it's probably the transmission slipping. If all other gears are shifting ok, you're about to loose the transmission.
Be sure the trans is full with the proper lubricant.
check fluid level, if good, will need pro evaluation
If it's only when you attempt third gear the problem is in your transmission.
Difficulty shifting, can't reach first gear and grinds to reverse, have to stomp on the brakes to downshift, basically any sort of unusual difficulty with shifting, car jumps forward when clutch is engaged or moves while on a flat surface and clutch is engaged.
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.
BAD CLUTCH