I replaced a broken pin in my Tmaxx trans and when I put it back together the output shafts(front or rear) don't spin when spinning the spur gear. Is this right ?
Which way are you spinning the spur? The Maxx's first gear has a one-way bearing in it, and will slip if you spin it the wrong way. Try spinning it away from the engine.
Does it have reverse? If so, then that is your culprit. The forward/reverse mechanism has a clutch in it, which allows the drivetrain to "freewheel" at low speeds.
Yup. That's due to the two gears on the bottom shaft (that the outdrives attach to). Those two gears freewheel on that shaft. They are kind of like the clutch on an engine, in a way. They have a single shoe that engages that metal barrel (kinda like a clutch bell), and that's what makes the truck go forward or reverse. The forward only conversion locks the gear to the drive shaft, eliminating the clutch setup.
I just did a forward-only conversion on my maxx; Olds is dead-on right. The trans will freewheel in both directions with reverse, but only in one way ("backwards") with forward only.
So long as you have a nice smooth gear mesh, you should be alright.