i kno how 2 check your oil ;D
With a special puller.
Of course.... the differential / transaxle uses a Mopar 75 - 90W Hypoid Gear oil
Trouble code p0731 means: A/T Additional Gear Ratio Error In First Gear
75w90.
P0731 Gear I Incorrect ratio
your shifter linkage is probably broken.
If a transmission speed sensor fails, the transmission will default to second gear.
When the trans computer has detected a problem it will default into 2nd gear. You need to have the system checked for codes.
You need to have the trans computer checked for codes. There will be a code that will lead to what is causing the no shift.
The transmission computer defaults to second gear when it detects a malfunction and sets a code. You need to have the TCM checked for codes to know possible causes.
It doesn't use gear oil. The differential is part of the trans-axle and it is all lubricated by the transmission fluid.
Transmission is in limp mode and most likely it is the input or output sensor or a wiring problem to one of them.