The viscous goes between the front and rear output shafts so under normal circumstances you won't be able to turn one without the other turning. However, if you lock one so it can't turn, then put a socket on a long breaker bar on the other end, you will be able to turn it very slowly against the resistance of the viscous unit. That is its function in life, to allow the front and rear propshafts to turn at a slightly different speed when cornering. Under extreme off road conditions it will also allow one end to turn while the other end isn't, so works instead of a locking centre diff with the advantage that it can't 'wind up' like a locked diff can.
The clunking you can hear will be the chain in the transfer box slopping about. If badly worn, it will jump a tooth under hard acceleration or if you are in low range going up a very steep incline or towing something very heavy. When that happens you get what is often described as a machine gun sound from under the car. Carry on using it like that and you will destroy one of the clutches in the gearbox so it's new gearbox time. If you can't make it skip when putting maximum torque through the transfer case, ignore it, if you can you need to replace the transfer box or rebuild it with a new chain. I could make mine skip last year so took it off and fitted a new chain, bearings and oil seals but that was after around 400,000 miles with a lot of that mileage towing. Chain isn't cheap though, https://ashcroft-transmissions.co.uk/product/borg-warner-chain-p38/ and a further £93 for the bearing a seal kit. Problem with fitting a used transfer case is you don't know how worn the chain in that is. If it has come from a car that has been used primarily on the road it should be fine but if the car was used for a lot of towing or off road it may be as bad as what you already have.
They are all identical though, 4.0 litre or 4.6 petrol and the diesel all use the same transfer box, the only ones that are different are those fitted to cars with a manual gearbox. Word has it that the ones on a diesel wear out faster due to more torque being put through them, so try to find one from a 4.0 litre petrol as that will have had the least torque put through it.