Having problems with gears

This sounds like a transmission range sensor either out of adjustment or faulty. If it has failed, here are the replacement instructions. There is a special tool that needs to be used for final adjustment after replacement, which I have also included a picture of.
Removal
1. Set the parking brake
2. Place the trans-axle range selector lever in neutral.
3. Remove the engine air cleaner assembly.
4. Disconnect the digital transmission range (TR) sensor electrical connector.
5. Disconnect the shift cable from the manual control lever.
6. Remove the manual control lever.
7. Remove the digital TR sensor.

Installation
1. Verify that the trans-axle range selector lever is in neutral.
2. Install the digital TR sensor and loosely install the bolts.
3. Using the special tool, align the digital TR sensor
4. Install the manual control lever and nut.
5. Connect the shift cable.
6. Verify that the shift cable is correctly adjusted.
7. Connect the digital TR range sensor electrical connector.
8. Install the engine air cleaner assembly.
9. Check for correct vehicle operation.

Could it be the linkage cable? I was told that as well.

If I can jump in here. it does sounds like the shift cable is worn out to me. here are diagrams below to show you how to do the job. Check out the diagrams (below). Please let us know what happens.

Absolutely, but make sure that the transmission range sensor is reading correctly if you continue to have issues after the cable replacement! You can do that with a scan tool that has the capability to read sensor parameters. I just say that because I have replaced enough range sensors to fill a 50 gallon barrel. Good luck.

Thanks for the advice. I'm gonna attempt to check it this weekend.

Awesome! good luck!