I've had zero problems running 85 octane in the last 10k miles or so (all 5k~10k' in elevation). Truck has 131k miles and I've driven from my parents ski cabin in MT to my home in N. AZ as well as to several skiing hills in CO (like Wolf Creek which is 10,300' at the parking lot).

85 is fine for the truck in good running condition (you might have something going on that's causing your problems, but it's not the gas).

Cheers,

Brian M
'97 T100 SR5 4x4, with Buckets