It's acting like your drive linkage is messed up so not a belt.
Rather than go through adjustment procedures the simplest fastest thing might be to flip the machine up onto it's bucket. Put down some cloth to protect the bucket. Turn the gas shutoff off.
Undo the screws that hold on the bottom cover and take a look. You don't have to be a motor head to understand what's going on in there. This explination may not make sense as you read it but when you look in there it will be very apparent what's going on.
The engine drives a belt which eventually turns a metal drive plate in the tractor section.
When you press the drive clutch the friction disk (the one with the rubber) in there meets the drive plate and that gets the wheels turning.
The shifter determines if you go forward or reverse and how fast. It does that by sliding the friction disk left and right across the drive plate. You go in reverse if the friction disk is on one side of center of the drive plate. It goes forward if on the other side. How fast is determined by how far away the friction disk is from the center.
Shift the shifter at the dash and watch how the frictin disk moves. What you should see in a properly set machine is the friction disk in first forward being (at the shift lever)close to the center of the frictin disk. Shift to second forward and it should move the disk a short distance further away from the center. Shift to fifth forward and the friction disk should move way over to the edge of the drive plate. Shift to first reverse and the friction disk should move to just beyond the center of the other side of the drive plate. Shift to second reverse and it will move a bit more further out.
What I described probably is not happening on your machine but it's what you should see. You can look at the linkage mechanism which will be simple. Look at the things that move when you move the shifter at the dash. It's only a few parts so something in there is probably disconnected.
The 824XL is a marketing name and covers many machines which have different components. Your specific machine is identified by a sticker at the back base of your machine. You can get that number and go to the Toro site and get manuals for your machine. The XL models are fairly new so owners, parts and service manuals will probably be available to download. If you need parts you can get part number and also get the procedure to set the linkage properly. When you find your problem you can reset the speeds to what you want by changing where the frictin disk lands when the shifter is at first forward. The other speeds will be relative to that. You'll understand that when you look in there.
