Have you seen this recall.
http://apps.hydro-gear.com/Portal/Do...1_20120125.pdf
I think they may be newer models but you could check your transaxle numbers. I have one old Craftsman with a Hydrogear that would not pull up a bank at the road at odd times. I could pull up on the brake pedal ad it would start pulling again, The brake was not fully releasing at the axle. I'm sure you have checked the free wheel linkage to be sure it's not partially engaged. Hydrogear has PDF service manuals available to download if you can determine which manual is for your transaxle.
Cannon