2000 Volvo 660 Model Detroit Engine

yes

 

I'd make sure the clutch is still bolted up and turning with the engine. If it is, then look up through the inspection hole and see if the input shaft is turning. If it is, drop the oil and pto cover and see if the drive gear is spinning.

 

I tend to agree.

 

Have you tried putting it in every gear in low range and seeing if it will move? Could be as simple as a busted 1st/reverse shift fork.

 

For the heck of it, put it in high range too and gently try a couple gears. Just let the clutch out gradually and if it starts to grip and begin to move, stop. Can cause some big(ger) damage if you try and lift off in high range. If it'll do something in high range, likely a range synchro issue.