You are out of line! gear lube contamination is easy to see, and an excellent primary clue for axle shaft (or bearing) wear. Cover removal is the first step to inspection, especially if shaft seals are not showing leakage.
It is a real inspection, and is a good sign. Finding a good mechanic and trusting him is important if you can't do the work yourself.
In a rebuild, never use a crush sleeve. Use a solid spacer. They are supplied as a shim kit. It makes future pinion seal or u-joint flange replacement much simpler, and the extra cost is insignificant.
As for the differential, forget about girly-man Kevlar clutch plates, or any clutch plates for that matter! Go with an Eaton Truetrac, and 31 spline axle shafts. No future worn plates, and no hassle with the pre-load spring removal if you ever need to remove the axle shafts.
Sometimes you can see metal, sometimes you can't. It's presence is a wear indicator, but absence of (visible) metal is not necessarily an all-clear. To some extent, a small amount of metal is actually normal in a gearbox or axle. It has some value as an indicator, but is not a definitive diagnosis. In the presence of other symptoms the OP has (noise, mileage) I say disassembly for inspection is warranted. This axle design's wear characteristics and life span are pretty well understood.
The solid spacer is reasonable enough.
As far as clutch type vs Torsen differential (Eaton True-trac) that is something of a personal preference. Both designs have their features.
The Torsen's obvious advantage is the lack of clutches with the associated wear and maintenance. Rather than describing the well-known Torsen characteristics I'll just
post a link. To summarize, they behave as an open diff unless slippage activates their torque bias.
A clutch type applies torque to both wheels and tries to make them rotate at the same speed. This has some advantages in that it applies a stabilizing force to a car, especially while under acceleration. A clutch type differential has a characteristic feel and behavior from the drivers seat.
Given the age and mileage of most Marauders many people have had an opportunity to decide which style to install due to axle regular maintenance. You might think there would be more people that went the Torsen route, as they are comparable in price to a new Ford 31 spline limited slip and don't need the clutches replaced when they wear out.
But despite the added maintenance many still choose the clutch type, myself included. I like the way they put the power to the ground and stabilize the car under acceleration. Is it better than a Torsen? Not necessarily. It depends on what you do with the car, and how you want it to drive. Personal preference. And in another 5 - 7 years I'll have to change my clutches again . . .
