Clutch slippage happens more in higher gears, due to the progressively less-favorable gear ratios as you go up in the gears. The higher gears give less mechanical advantage to the engine, therefore higher torque loads across the clutch, and a worn clutch can't handle the higher torque. A clutch can also slip because of getting oiled- a bad rear main seal or trans input shaft seal on a stickshift car can be bad news for a clutch.
If you can make the clutch slip in 3 & 4, you can be sure it'll slip in 5th (OD) as well. With clutch problems already existing I wouldn't worry about the OD until the other issues are fixed. However, while the trans is out, that's a good time to change the OD fluid as well as the gear oil.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 221K, 88 744GLE- 202K, 91 244 181K, 88 244GL 145K