I missed where you said it has never been serviced. That tranny, as most, needs servicing every 30,000 miles or you will kill it.
So back to your other statement regarding the fluid looking fine, it can't look fine or you just didn't know what fine should look like. It should be a bright crystal red with no brownish coloration or burned smell.
Do you know how to check and or refill that tranny, if so tell me how you do it, want to make sure since it is a sealed unit without a dip stick.
If the fluid is at all brownish, you will need to do 2 drain and refills adding a bottle of Lucas tranny additive on the second refill.
Curious, since you have not been following your maintance book, have you services all the other fluids yet such as the p/s, brake fluid, both diffs and the t/case if not go read my major service list posted at the top of this section and consider doing it all including the front drive shaft.
If you need any help shout.
