I am not sure about whether the steering gears are interchangeable, but I know they have some definite differences. The steering gear was changed for 05 because of some problems with twisting the shaft. Ford decided to increase the size of the shaft to combat this problem, so the 05+ has a larger diameter shaft. The problem is they left the gear set exactly the same so now the shaft doesn't twist, but the gears break in the box.
I am on my 3rd steering gear on a stock 07 F250 4x4 with 75K miles because I have broke gears in two boxes only with routine driving. I would much rather have a twisted shaft since broken gears binds up the gears equating to NO STEERING!
I would recommend contacting Red Head Steering Gears in Seattle to ask your question. They know all about the different gears and problems/advantages of each.