Shift Issues with Yukon xl 2500 automatic

I have a 2000 Yukon XL 2500, 6.1L, with an automatic transmission that shifts hard only after extended driving. When driving in town i expierence no unusual shifting, however, when driving on the highway for extended time, more than 2 hours, the upshifts become very hard. I bought the truck used last year with about 112,000 miles on it and now have about 132,000 miles on it. The only services I have performed on the truck have been oil changes, air filter, and rear brakes. Any help would be appreciated.

This may just be in need of the normal trans service, or it could be a problem in the electronic shift controls, a full scan with a trans function by a dealer or trans shop should help pinpoint the problem.