There are some very strong laxatives out there, which don't even require a doctor's prescription. However, I would be careful of taking anything when there are uncertainties surrounding the state of your intestines.
When I need to I take lactulose, which seems to be very strong. I usually have the opposite problem - diarrhoea - which most people with Crohn's and colitis have, but there have been times when medications for other things caused severe constipation. Stimulant laxatives are a bit different from stool softeners as they provoke your bowel to move, whereas stool softeners just draw more water into the stool so when you do go it's less hard and painful.
The Wikipedia page on laxatives is a good source of helping you figure out the different kinds of laxatives and which may suit your problems:
http://en.wikipedia.org/wiki/Laxative
But it's best to know what's causing your constipation before deciding how to treat it. How long have you been constipated for and is this normal for you? You've only had these symptoms a short while - did you have any other digestive problems prior to this? Crohn's and UC (though it sounds like you haven't been conclusively diagnosed yet?) are chronic illnesses, and testing to figure out the diagnosis and treatment are long-term processes. It is difficult when you want immediate symptom relief, but you'll need to be prepared for the fact that answers may not come straight away.
If the constipation is accompanied by severe stomach pain, vomiting or fever it might be worth you visiting the emergency department at the hospital. If you have a blockage, the pain and other symptoms would usually be severe enough that you'd know something was very wrong. If you don't have any other symptoms and the pain is manageable, I would say check with a doctor about ways to treat your constipation, though I know that sometimes getting to see a doctor immediately is not always possible, and I've also found that sometimes treatments doctors recommend aren't necessarily any better suited to me than the times I've found medications for myself.
If you can't get help from a doctor and want to treat the constipation yourself, my advice would be start with
very small amounts - no matter how much you want relief, many laxatives are extremely strong and you don't know how your body will react to them. Also some laxatives can dehydrate you, so you need to drink a lot. Some can be dangerous if used long-term. Also you said you're mostly on a liquid diet - if you're not getting enough fibre, this may not be helping things. (Personally I've always found fibre made me worse - a fibre supplement was an example of when a doctor proved he had no better understanding of how to help me than I did myself.)
I hope you get some relief soon.
