Hi. If your C: comes "before" D:, then you need to defrag your C: before you can shrink it.
Not sure how the partitioning programs works but this makes sense so sounds to be worth a try atleast.
I'm fairly sure you can only extend partitions to the right and not the left. Not entirely sure if thats true but I would just use some freeware program and do it through that if so.
It could be fragmented with files on that part of the drive so if a freeware program would be willing to partition it the OP could lose data.