Truck Shiftin' Hard???

Ive got a 99 sonoma with the vortec 4.3 v6 with around 85,000 miles. when i drive my truck for a bit and it gets warmed up, it starts shifting hard into gear. does that sound like a gearing problem, tranny flush, etc??? what do u guys think it may be?

dont flush it just change the filter. if that dont help its probly a shift valve. I work for a gm dealer and see lots of prob with the shift valves and people flushing

anyone else got any ideas. is the shift valve an easy fix or does it require a professional fix?

If your not scarded to take the pan off then most of the shift valves and be had from there some you have to drop the valve body to remove though