Tricky to answer without being there. With the engine off, push both levers forward and see if they stroke or move the same amount. In other words in neutral they should be evenly spaced from you forward and backwards, and when "against the stops" the should be even also. also check the servo lever on the pump ( the arm your linkages hook to) and see if they look like they are stroked the same amount. If they are stoping shorter or longer on one side don't turn the clevis on the rods. that only shortens the travel one way and lengthens it the other. there should be a stop in the pivot mechanism for the handles, one for forward one for reverse per each side. These are how you adjust your travel handle stroke distance. Now then.... that is just if it is a handle issue. When you run your machine if the deviation from straight isn't bad, or it is a small amount over a long distance, this could be normal. However, if it veers off bad, or it gets worse when the oils warms up, you probably have hydrostat issues. Warm your oil up until it is uncomfortable to hold your hand on the side of the tank and then put your bucket up against something very strong (concrete wall) and ease your controls forward. If everything is normal, both tires will spinout evenly. If you have a problem one side will spin out and the other will do it less, or may stall. You are looking for both sides to have equal strength. With out gauges hooked up this is the best "seat of the pants" test. let us know what you find.
