1st of all... while engine is idling, on a level surface, and warmed up. Checked the trans fluid level. It should be in the crosshatch section of the dipstick.
1. Look at the color? How does it look? Is it Red? Or Brown?
2. Smell the fluid? How does it smell? Burnt?
3. Use a clean white tissue or paper towel... wipe the dipstick and reread the stick a few times to be sure of getting an accurate reading. How the the fluid look on the towel? Does it wipe off Red, Grey, or Brown?
If it's low add ONLY the appropriate fluid. It's listed on the dipstick. If the level is at the LOW point of the crosshatch marks, add a PINT. = HALF a Qrt.
If the level is at the tip of the stick, add 1 Qrt. No additives.
Once adding, you have to let the vehicle continue to run and the dipstick area to completely drain back down. Then recheck your fluid level.
Post your findings... color, smell, dirt level.....Also, if you want to keep your vehicle running, stop driving it until problem is fixed... or you're looking at $$$
PS, I know you said you checked the fluid, but some people don't know how to check it properly... plus the added info I asked also provides trans condtition.
PPS. Another possibility, if all else checks well... the shifter cable maybe loose or the bracket holder for the cable may have come undone. Hence no driving issues once in gear.... only manually switching gears. This is located on the top of the trans, usually just below the Master Cylinder for the brakes. Black Triangular shaped sensor (Range Sensor) & the cable attaches to the top of it. This is a pic of a
similar trans, in order to give you a visual reference.
https://www.dropbox.com/s/9g4cazub1t...44.30.jpg?dl=0