Charlie is right. Don't understand the connection between changing some driveshaft parts and now it won't go forward. It's like saying " I bought a new pair of shoes and now my air conditioning isn't working."

Give us more info. Did the truck have all it's gears when you pulled in to change the stuff? Walk us through every step of what you did.

Something is missing from this picture. You say you have three on the column, have someone lay under the truck (engine off........blocked up safely) while you shift through the gears. You should see each arm on the trans as it moves from one position to the other. Particularly pay attention to the one for reverse/Low gear, as that is probably your culprit. Did you try starting out in 2nd or 3rd to see if those are ok?

Just give us more to work with and someone on here should be able to spot the thing that went wrong in the repair.

Don