The clunk wasn't it slipping out of high range into neutral by any chance - my old series car used to do that occassionally as did my disco from time to time.
 
Not a likely cause but it is so easy to check just in case it is it - just move the lever in an out of high/low range a couple of times.
 
Clunks otherwise could be bang in the rear diff - try to to drive in 4wd = axle busted?.
 
Bangs in the gearbox will still normally result is some ability to proceed.  
 
You mentioned an O/D - I assume there might be some sort of false neutral like the H/L range - but if gears damaged I would have though some sort of movement might still possible but then I am not familiar with them. Try engageing the drive train while the car is moving (in gears are stripped and moving gears to some part that still has teeth - try changing the O/D to another position.
 
Just read that the handbrake not hoilding - I assume it worked before so that points to a busted axle, drive shaft or diff - engage 4wd and see if the handbrake then holds.
 
Some options to consider.
 
Gazzz
				
			 
			
		 
			
				
			
			
				REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101 
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
			
			
		 
	
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