Scottrh,
I had the same problem as you since I also have a 2520, the mower deck and linkage would go up and down with the use of the TPH, I wrote about it on 11-18-2007 under the title "Turfs, 62D and chains", you can check it out if you want. The part number you need is LVB24962, cost me $151.50 to get the "Lock-up" kit installed, and it works great, keeps the mower deck up while using the TPH.
Another thing to check is your TPH draft arms since you have a 2520, are they real close to the rear tires or are they the newer type which gives plenty of clearance. If your tractor is still in warranty, you may want to check into that.
Anyway. hope this helps and welcome to TBN!
Larry