I'd check the gearbox output shaft for wear, if you have the old non drilled input gear in your LT230 it wears the splines of the gear box output shaft and gives a big old clunk when it takes up slack when you put in reverse.
It's quite easy to check as there is a plate on the back of the lt230 you can remove if there is a load of gungy crap inside the gear and it moves on the splines that's your clunk.
Edit: I'm assuming you have an R380 and LT230 ... I don't know if this issue affects earlier gearboxes