I noticed that this has gone unanswered for some time, so let me have a quick whack at it. First off, I am not a rat geneticist. Rat knockout models are only recently becoming more and more available. That being said, I would highly recommend that you change your animal model to mice if at all possible. Many labs around the world have already developed the animal models you would need to create a conditional knockout of Vit D. receptors in multiple tissues. 

I would suggest for you to look into the Cre-LoxP system. In essence the expression of an endonuclease (Cre) is driven by a tissue or cell-specific promoter. Many such animals have been created and can be found by contacting either a mouse bank or an investigator that created the Cre-driver line. There is even a Cre that will only work when a drug such as Tamoxifen is given to the animal CreER. When Cre is activated, It will remove a gene or portion of gene flanked by LoxP sites. You could search for such "floxed" animals to cross to a Cre-driver line. 

I do not mean to discourage your investigation in Rat, but transitioning your model to mice would likely open your options considerably and prevent you from having to create three to four rat models.

Good luck

In continuation with Justin's suggestion, you may contact technical service of jackson laboratory, USA, they will advise you further. Also go to MGI or "mouse genomic information" site on google, enter the gene name, you will get preliminary idea, what is available.

Thanks for valuable information...