It's a common misunderstanding that meat from the hindquarters of an animal cannot be kosher because it is the part sacrificed to God in ancient times. Actually, the designated burnt offering was the fat inside the abdomen, around the stomach and kidneys. All the meat of an animal acceptable under Jewish dietary law can be made kosher, but the process is long and complicated. If you are determined to serve a rump roast or a T-bone steak and still maintain a kosher kitchen, it can be done, but it'll cost you in time or effort, or both.
Make sure your hindquarter has been slaughtered according to kosher rules. This whole process, from beginning to end, must be supervised by an expert called a mashgiach. If he finds anything he isn't sure conforms to the Laws of Moses, he calls in a rabbi to give a ruling. If you are doing this yourself, you will need to consult a mashgiach, who usually works for a large firm, but some will take special private jobs like yours.
Kosher your hindquarter by removing all the blood vessels, "unacceptable" fat and other required parts such as glands -- this demands the skills of a neurosurgeon. The forequarter (anything forward of the tenth rib) has fewer blood vessels and lesser fat than the hindquarter, so it's easier to "kosherize." Soak the meat in cold water, salt it thoroughly and drain to remove as much of the blood as possible, then soak it again. If you can't get this done right away, wash the meat every three days until it's koshered, and be sure to eat it before Day 13 after slaughter or it automatically becomes "treyf," or ritually unclean for religious reasons and therefore non-kosher.
Remove the sciatic nerve from your hindquarter. There's an Old Testament prohibition against eating this nerve that seems to be related to Jacob's injury during his wrestling match with the angel. Since this nerve is very deep in the hip joint of the hindquarter, it's hard to get at and must be removed by a special process called "porging"; not many people know how to do this, so the hindquarters usually get sold to a non-kosher butcher.
Re-evaluate your hindquarter of beef. After all this washing and salting and soaking, pretty much all the natural juices have been eradicated from your rump roast and your T-bone is a pale shadow of its former glory.
Braise your depleted hindquarter cuts by searing them and then cooking them long and slowly in a small amount of liquid, like pot-roasted brisket.