No, there are no freeware DTS encoders. The least expensive one is the vortex surround encoder (NOT the "Zoom" encoder)
http://www.vortexzoom.com/tag/vortex-surround-encoder/but I'm not 100% sure that does 48 KHz as I don't seem to have a copy anymore.
Your best bet for high quality is rip your cd to wav, re sample to 48 KHz 16 bit (You'll need to check off "bitrate to" and select 16 bits, in the current version of audiomuxer, to keep the original bit depth of 16) with audiomuxer and use the resulting wav files to make the disc. That will be 48 KHz 16 bit PCM which will work in any DVD player and will be the highest quality guaranteed to work. The down side will be the file size will be ~700 MB per album.
Better quality still if you get a copy of r8brain pro, to do your sample rate conversion (That's vs. the free version of r8brain that audiomuxer uses).
Anyway, starting with lossless rips (44.1 Khz 16 bit) you can experiment with the different lossy encoders (AC3, DTS) and bitrates to decide what you want, in a trade off of quality vs. # of songs per DVD.