I have a craftsman DGS 6500 garden tractor, it starts and runs fine, the blades engage and are working fine, however the tractor moves very sluggishly, if at all. I have already purged the transmission, and the power engage rod is pushed in completely. What else do I need to check and how?

Make sure that the brake is fuly released. Many times after being parked for a long time with the brake on it does not release properly. If that is not the problem then you can try to change the fluid in the transaxle. Since you already know hw to purge it that should be firly easy. I believe that the transaxle calls for 20w50 synthetic motor oil. You better check on that since I am not sure.