Di fretta = precipitosamente. Hastily. In a hurry.
In fretta = quickly, rapidly.
Vado di fretta = no time to stop and talk.
Vado in fretta = I am a fast walker/driver.
Un lavoro fatto di fretta = a job done hastily (that is, not really a good job) because, say, it's due very shortly.
Un lavoro fatto in fretta = a job done quickly.
It's really just a nuance. Anyway, I hope it's a bit clearer.
