I know what you mean about the $$$. If it were me though, I'd bite the bullet, do the pinion seal AND the axle seals, drain and clean out the housing, put a good seal on the differential cover, and fill it with the best gear oil I could get my hands on. If you're doing the pinion seal anyway, the axle seals aren't all that much work, and cleaning out the housing (don't use solvents, just wipe it all out) is a no brainer if you have the cover off.
You are going to want to use the oil that Ford specified, as well as the correct friction modifier. You have a clutch-type L/S (limited slip) differential, and the lubrication properties are critical to it's correct operation. Putting the wrong stuff in there will either:
1) Cause the L/S to act like an open differential and give you less traction.
2) Cause the L/S to grip too much and wear out prematurely, then acting like an open differential and giving you less traction.
You also might check to make sure your axle breather tube is clear from any obstruction. It that has gotten plugged, the oil may be burping past the seals when the axle heats up and the air inside expands.
2003 Ford Ranger Edge 4x2
3.0L, 5 sp, 28 spline 8.8" with Lock-Right, 85k Miles
31 x 10.5 R15 BFG Radial Long Trail T/As
Hopefully soon to be 4x4
Anyone need to get rid of stock 4WD Spindles, CVs, Front Diff (4.10), Front Dive Shaft, T-Case, or 4x4 ABS Pump?