Since its a dual-boot with one OS accessing the partition at a time,
TrueCrypt should have no problems.
All you need is, to isolate the data partition and encrypt it as a volume (rather than the files in it).
Your idea of a
D: drive for Windows will work well with a mountable partition for Ubuntu.
After that you need to get the platform specific TrueCrypt binaries on Windows and Ubuntu.
Whichever OS you boot into, just mount the encrypted volume and you are good to go.
This will work and is easy because,
- TrueCrypt is available across platforms
- You are dual-booting and not even sharing the partition simultaniously with Win/Ubuntu
- If you were sharing, it would typically be over network (and the host OS would mount it)
- You do not seem to be oriented towards trying to encrypt the boot partitions
- which can also be done (independently for each OS, if you so desire)
I do this regularly with a removable USB drive.