Jhicks427,
Since you readjusted the lap lever first (which is what I would have done) and it made no difference, I would check for leaking seals on that drive. If you discover same, the drive requires a refill, at the least, and perhaps new seals and gaskets.
Next would be a tear apart of the drive to see what the internals look like. I have read that small metallic particles can plug various fluid valves in the open position, lowering the speed and power of that drive. A simple clean out was all that was required for a fix.
All of that said, you can find your drive make and type (Hydro Gear, etc.) and serial numbers on the body of the drive. A search online should find the correct repair manual in pdf format or as a zipfile. I was able to find the drive manual for my ZForce 48 back 6 months or so ago.
Some drives are rebuildable, if you have the tools, the shop, the time, and are a reasonably skilled backyard mechanic. Some drives are simply throw away's, as in many RZT's. You won't know which you have, until you find the drive specifics (model and serial #'s) and do some searching. If you are a DIYer is the next answer. After all, it doesn't work now, so nothing's lost from trying and not being successful.
Short of that, your next step is eBay, parts suppliers, other independent small engine shops who scavenge old units for parts (as many makes use the same drives), etc., or going new from Cub Cadet.
Good Luck!