Once you've captured/recorded a distorted signal...ain't a whole lot that can un-distort it. Re-recoding it is the only real option...IMO.
Good Panning will allow you to have a lot of different elements in the mix without them all fighting for the same space. Also, getting the right EQ balance for all those different elements is equally important. If your Vox are deep, male vocals...and you have a lot low end instruments...then they tend to mask each other.
With most lead vocals....I Pan them dead center...and I never have anything else occupying that exact same space. Even with Bass guitar, I'll Pan it just a little off to one side, especially with baritone male vocals.
Muddiness is a result of poor mic technique, where someone "ate the mic" while they were recording, and the proximity effect made the Vox real boomy/muddy...or sometimes the preamp and anything else you do to the signal can make it muddy. Sometimes the room you record in will contribute to that.
With already recorded muddy vocals...you can scoop out a little of the low mids (for male vocals, somewhere around 200Hz-400Hz)...and add a touch of mid-highs for clarity/crispness (around 4kHz-6kHz...just watch the SSSSSS-sibilance around 6kHz-8kHz).
When you record vocals...have the singer step back a good 12"-18" away from the mic...and just bump up the gain a little on the pre if you need more signal. That will keep the vocal more balanced sounding...but sometimes it also depends on the mic you are using, as some will be a bit muddy…which is why you always want to have at least one high-end mic in your arsenal.
For spoken Rap…maybe something like an announcers/radio mic would work well (Electrovoice RE20)…?
http://www.zzounds.com/item--ELVRE20