You said the tranny fluid looked and smelled OK, but did you check and smell the front diff grease? That could have gotten hot under the heavy continued load, or possibly you've got a bad axle boot that could have slung axle grease on the exhaust system.
Just some things to check.
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Huck
Subaru Ambassador
92 SVX LS-Tour Magnaflow Exhaust, 5-Spd-AWD
88 XT6 AWD 5-Speed "Bride of FrankenWedge"
15 Impreza Premium Sedan
15 Crosstrek XV 5-Speed
My 5-Speed "How-To" Write-up
1976 Pontiac Firebird Formula
Current Count of Subaru's Owned.... "70"