Bad buck in fourth gear

I have a 99 suburban 4X4 and when I am driving over 50 MPH and I go up a hill at a steady speed (with or without cruise control) and I let my foot off the gas slightly the truck bucks really violently. The transmission was serviced recently and they said it was a spark/ignition problem. So when I took it in for that they replaced the cap and rotor and wires and fuel filter and cleaned the fuel injector, but no luck. Please let me know if anyone has had this problem.

It feels like the truck has binds up.

One mechanic made mention of the 4th gear lock up feature to the transmission, but I don't understand.

Hey my friend I have the same problem and it just start yesterday, Suburban 99 4x4 77k miles and it happen when is over 55 and it shift to over drive. I need help before the dealer will get my $$$$$