Dear Vimridge
As far as I can tell, (person who usually does support on Rentrak is away) there is no way to modify the spec to only include rental lines (there is no spec code specifically for either rental lines or expendables and sell-offs). The spec code for .D lines prints all detail lines no matter what type of "line" it is. I tried this here on sample data.
If the information obtained from the "Rental Report" or the "Print Contract Detail" (both found in the Reports | Print Management Reports) is not enough, you could use Quik Reports to create a custom report, if your need for this type of document is important enough. This way, you can limit what shows up on your report by "line type" excluding all types except "IN" or "OUT".
I hope this helps. Sorry it is not the answer you would have hoped for.
Regards