1990 Ford F-250 Strange Smell

This has been going on for over a year now. The issue is I get a strange smell which is very similar to burning brakes after going down hill over a period of time. I had new brakes put in which didn't fix it and then I complained and Les Schwab replaced the brakes and put an entire new set in. I never notice the smell around town. I only smell it after being in the mountains and am driving back a prolonged down hill road. It might only take 10 to 15 min and the smell becomes overwhelming. I have the transmission in drive and am really only taping the brakes. I am going slow too 20 mhp or so. Although I have had it happen driving down a pass doing 60 mph too. The engine is running fine and so is the tranny. All the fluids are new too, In fact I had the tranny fluid completely changed 1500 miles ago. It never happens going up hill or driving around level ground. I let it sit for 15 minutes or so -to cool down after this happens and then all is good to go again. My next test was going to put the truck in neutral just to see if something is causing stress on the tranny. The tranny is a E4OD the engine is 460