Need Help with Transmission Shifting Issues

[kna4977]

Have an 05 Yukon with the 4L60E that has recently started having issues in the 3-4 shift. All other shifts seem to be normal for now. When it shifts from 3-4 it seems to engage, disengage, re-engage. Does this cycle fairly quickly at least twice before it settles into overdrive. Usually can be mimicked by starting onto an interstate where it has to go through all the gears under normal acceleration. Can see it do this by watching the RPM gauge fluctuate up and down as well as hear and feel it. Another strange thing it will do is around 45 mph when it can't fully decide whether it wants to be in 3rd or 4th it will shift into third then klunk into lug in fourth. Any ideas as to what is going on and how it can be fixed would be great.