A gear oil leak around the base of the pinion flange on the front or rear differential (or even the transfer case) is a sign of a leaking seal. You shoould first check the the axle breather tube is free and clear, if not it can allow pressure to build up inside the axle and force oil out past the seal.
First step in removal is to loosen the four "companion flange" bolts that hold the driveshaft to the pinion flange. Use a pair of long handled 14mm wrenches to break the nuts free and remove the 4 bolts. Set the end of the driveshaft to the side, being careful not to let it separate at the slip yoke.
Check that the pinion flange nut is tight, if not, it can cause the pinion flange to move around under load and cause the seal to leak. If it is loose, tighten to ~90 ft.lbs. torque with a 30mm socket and torque wrench.
If the above checks are fine, then proceed to replace the pinon seal, see below for the replacement steps:
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