I have a john deere stx38 , has difficulty climbing inclines or hills, runs fine on level ground.
Have replaced engine mount bolts
Have replaced drive belt
Have checked fluid level in transmission
Note this is a hydro drive

Question: is there another adjustment I can make or is there something else I should look for?

Hi,
STX38 Hydro has what is considered a sealed, non-serviceable transaxle. That is in no way to say it true, but you would need to know the principles and service techniques involved. I have torn them down and generally it has about a 50% rate of serviceability.
Peace,
Clarke