the Deere does not have MFWD just 2wd but it is a strong 175 or so we usually pull it around 4-5.5 mph and only 4" or so deep in soybean stubble , otherwise it will bury itself and all the trash too. in cornstalks we usually try to go 6" deep and 4-4.5 mph, How deep do you go in wheat stubble?
I don't think we have ever run a finisher through wheat stubble. We usually chisel plow our wheat stubble in the Fall. The tractor will pull at the same speed in chiseled ground, as well as soybean stubble. In chiseled ground, our finisher will go down 4-5". In optimum conditions, I can pull our DMI chisel plow(7 shanks and 2 ripper shanks) at 5 mph. If conditions are right, it will go 10-12" deep, with the ripper shanks going a bit deeper. Of coarse, it all depends on the right conditions. Last Fall we had good chisel plowing conditions, but it was a bit slick on top. To not loose traction, we had to not go so deep.
I'm a Massey fan, but the style your JD
4650 is, is probably my favorite of the JD
tractors. Do you have any pictures of your rig?