GO YE UNTO YOUR BURDENS — Go to your work which you have to do at home; but it cannot signify “go to your labours as slaves”, for he was speaking to Moses and Aaron who were of the tribe of Levi, and the work of Egyptian slavery had not been imposed on the tribe of Levi. You may know that this was so, because Moses and Aaron went and came just as they pleased (Exodus Rabbah 5:16).
Why weren't the Levi'im enslaved like the other tribes? Is it that they were already serving as a kind of priestly tribe though this title was moved from the b'chorot to the tribe of Levi only later on in the Torah and it's a case of אין מוקדם ומאוחר בתורה? If so, a. What duties exactly did they have at the time (with there being no Mishkan)? b. Why should Pharaoh even care that their tribe had a different status among Bnei Yisrael, enough to respect it and not enslave them?
If it's not a case of אין מוקדם ומאוחר, is there a different explanation?