Yes, you can buy just the hitch shank to fit your 2-1/2" reciever and use your existing weight distributing hitch head, bars, etc with the new shank, provided you get one that has a 2" square vertical bar and the same vertical hole spacing and diameter as your existing 2" shank.
The 2-1/2" to 2" adapter tubes are sold at Tractor Supply, Harbor Freight, some auto parts stores and most online hitch/trailer parts supply stores. Or, you can buy a 2" receiver tube (also sold in the same places), cut off the reinforcing collar, and use it as a reducer tube.
Also, most metal suppliers sell 2-1/2" sq x 1/4" wall seamless tube, which is what is used to make 2" receiver tubes and reducer tubes...
