Transmission only operates in one forward gear,1991 JKG with 4.0 engine

hopefully maybe itz low transmission fluid...chek of course when hot and engine idleing...

1991 GEO STORM 1.6L 4-cyl Engine Code 5 DOHCCAPACITIES
Engine, with filter.......... 3.2 quarts
Cooling System, W/ MT Initial Fill.......... 7.4 quarts
Cooling System, W/ AT Initial Fill.......... 8.0 quarts
Automatic Transmission, Initial Fill........ 4.3 quarts [1]
Automatic Transmission, Total Fill......... 7.0 quarts
Manual Transmission, ..........4.0 pints[1] With ENG at operating temperature, shift through all gears.
Check fluid level in PARK and add fluid as needed.1991 GEO STORM 1.6L 4-cyl Engine Code 6 SOHC
1992 GEO STORM 1.8L 4-cyl Engine Code 8 DOHCCAPACITIES
Engine, with filter.......... 4.0 quarts
Cooling System, W/ AT Initial Fill.......... 8.0 quarts
Cooling System, W MT Initial Fill........... 7.4 quarts
Automatic Transmission, Initial Fill........ 4.3 quarts [1]
Automatic Transmission, Total Fill......... 7.0 quarts
Manual Transmission, .......... 4.0 pints[1] With ENG at operating temperature, shift through all gears.
Check fluid level in PARK and add fluid as needed.

Check the transmission for leaks and fluid level. And fix as needed. However, seems like your transmission is slipping.

This is what your codes mean:

Trouble Code-P0734 (A/T Additional Gear Ratio Error In Fourth Gear)

Trouble Code Conditions:
Vehicle driven to a speed of 4-37 mph at more than 500 rpm, gear selector in 4th Gear in Drive, and the PCM detected too much difference in the engine speed and output shaft speed.

Possible Causes:

• Customer may complain of intermittent 4th gear operation
  
• O/D and 2/4 pressures less than 75 psi
  
• O/D or 2/4 clutch volume indexes out of specification
  
• O/D accumulator seals are damaged
  
• Transmission enters "limp in" mode running in 4th gear
  

Trouble Code-P0735 (A/T Gear Ratio Error Fourth Prime)

Trouble Code Conditions:
Vehicle driven any forward Gear, and the TCM detected the ratio of the Input speed to the Output Speed did not match the current Gear Ratio (this test can take up to 5 minutes).

Possible Causes:

• Related Gear Ratio trouble codes may be stored (note that some of these Gear Ratio trouble codes may be intermittent)
  
• Transmission has internal problems or damage present
  

You have a transmission problem.

Do you have a check engine light on? Is the engine getting to operating temp? Is air filter clean? Is the fan clutch free? Does engine idle and run smooth? Does the transmission have all 4 forward gears including tcc engagement? Are the tires the correct size?

Mke sure that the transmission is working. you can tell if the engine rpm changes when you put it gear (forward or reverse) if it does not then you need to check the level of the transmission fluid for an automatic or you may have clutch problems if it is a manual transmission. if you have more details on the transmission type (auto or manual) and if it is doing anything when you try to get it to move that would be helpful.

sounds like transmission damaged. if forward gears are operational, suspect transmission failure. If nothing working ( either in 2 or 4 wheel drive) suspect transmission or transfer case.

Was fluid and filter changed to fix this,?????????? or did it happen right after this was done??????????. you are not clear.

transmission is bad

You have a transmission fluid pump for the reverse gear and a 2nd one for all forward gears. Something is wrong with the pump. Your transmission problem could be the result of neglect if you haven't been flushing your fluid at least every 40,000 miles. Flushing the transmission now may fix the problem, it may make it worse, or it might not make any change at all. The first step is to find out that your transmission fluid level is correct by checking the dipstick with engine at operating temperature while idling in Park. DO NOT OVERFILL. NOTE: your fluid doesn't get thin when it gets old, it gets THICK and dark due to contamination. The darker it is, the longer it's been siting there.