I just checked the prescribing info again and it does say Diazepam can cause constipation
http://www.drugs.com/sfx/diazepam-side-effects.html
It also says there can be withdrawal effects if stopping it quickly so you may want to taper down as much as possible when you come off it.
I know you really don't like having an enema, or taking laxatives, but if you did take laxatives it would only be for a short time and if it prevents needing an enema then it's the lesser of two evils. If you end up needing an enema I wish you all the best and hope it is as comfortable as possible. The relief afterwards is a pretty good thing in my experience.
Hugs.