Have you checked the trans oil and filter? is the oil burnt? Check the main pressure in neutral and all gears forward and reverse. Do this with the oil and trans at operating temp. Also check the converter and lube pressure. You can measure this at both idle and at 12-1600 rpms.
It is possible that the gear selector or circuits could be faulty and not sending current to the neutral/drive coil but it should not creep in forward if that is the case. More than likely it is a bad forward clutch pack or seals inside the transmission
Hi i agree with Ben in regards to checking oil and filters etc. Depending on what you find would determine if you have to remove the transmission for repair.
If the condition of the oil and filters is ok i would do a few simple checks
1/Assuming that the main system pressure is OK check the forward clutch pack pressure, should be approximately 189-241 psi at high idle. If this is ok it would suggest that the forward clutch is not engaging due to wear. low pressure at the forward clutch would indicate worn or damaged piston seals or worn or damaged rotating shaft ring seals
2/if there is no forward clutch pressure, check that there is power to the neural /drive clutch solenoid #1,
NOpower to solenoid #2 in the forward direction, if there is you need to find out why.
3/If there is power to S1 and no power to S2 it is possible that the forward/reverse slider valve has jammed in its bore as shown in the diaghram below.
if you find that the valve is damaged it can be interchanged with one of the other positions as they are all the same
Mark