floor = transmission tunnel
when you put it in 4 lo, the shifter is forwards, and may hit on the edge of the transmission tunnel hole where the shifter goes through to the gear box.
even if the shifter doesnt hit the tunner or anything when stationary, when you are driving, or under load, the engine and gearbox will move due to torque, and the movement may be enough for the shifter to hit enough to pop back out.
do you have any body lift? are your engine and gearbox/transfer mounts all good still?
if all the mounts are still good, you can take the shifter out of the transfer, put it in a vice, and bend it backwards a tiny bit so it clears.
and as mentioned already, try checking the shifter boot also. this may be bunching up when in 4 lo and pushing the shifter back.