what is the cost to rebuild the front diff?
ENORMOUSLY more than finding a "good used" differential assembly with the correct gear ratio from a Treasure Yard, slapping in a couple of output seals, and
maybe the pinion seal.
Do they usually replace the gears and axels?
Not unless there's something wrong with them, including the desire for a different gear ratio.
Vehicle is a 1999 tahoe 2 door and this thing has seepage from everywhere. I am not interested in rebuilding my self. I don't know if I could even do it.
Define "everywhere". The usual problems are the left and right output seals. SOMETIMES, the pinion seal.
If the sealant used between the case-halves is leaking, though, you've got real trouble.
Make sure the vent hose is properly attached, not kinked, and has no holes in it.
Assure you have the CORRECT amount and quality of lube inside--probably by draining completely and re-filling with the specified amount.