Shift Issues with Yukon xl 2500 automatic

[speedway1839]

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.

[maxwedge]

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.