Get a big screwdriver (playing "Primitive Pete music, here) and pry the u-joint out of the diff trunnion. You don't even want to try and pull a u-joint underneath the truck, believe me! Your front spline is probably just caked with dry dust-filled grease. You can raise the truck to help get the driveshaft out, but when you go to install the center support bearing onto the frame, you must have the wheels on the ground with the truck's weight on them (or jack stands under the axle) so that the center support is aligned properly fore-to-aft. Otherwise the bearing can pop out of the carrier. AMHIKT.
