Truck lunges into second gear...

I have a 1996 GMC Sierra 4x4 that I recently purchased(about 5 days ago) I bought it from a used truck dealer. Yesterday I experienced the following problems and I'm looking for verbage, suggestions, or ideas on what this could be. The vehicle I'm referring to is an automatic with 133000 miles on it.

When sitting at a stand still and then accelerating the truck takes off fine but when the transmission shifts into second gear the truck has a hard lunge feeling.(almost like being hit from behind) It only does it between first and second gear. Yesterday was the first time the truck has done this since I bought it 5 days ago. I also was able to make it stop doing it...by going over a speed bump at a fairly fast clip...I know that sounds stupid...but the bumping seemed to correct it...cause now its not doing it...could a plate or something in the transmission housing be sticking or something...

Thanks for any input you can provide

I have a 97 Bravada that does the same thing.
You should have, or will soon get, a Service Engine Soon light.
The problem (with mine, and I assume yours) is one of the valves that controls the Torque Converter Clutch.
Depending on who you talk to, it it one of valves, but the result is the same.
The valve sticks, and the TCC does not engage. The computer reads this as a slipping transmission component, and commands full hydraulic pressure. This causes the harsh 1-2 shift.
After 3 (or 4) consecutive occurrences, the SES light will come on.

Try this: note when it happens, with me it is after driving for 20-30 minutes, at more than 45 MPH.
Keep the transmission in D, or 3, instead of OD or 4, drive the same way, and see if it does it.
You have to shut the engine off to get it to reset once it does it the first time.

First option: take it back, and get your money refunded. SOMEBODY knew it was doing this, and reset the computer to sell it, and/or to get you to buy it.

Second option: have a trans shop fix the valve. This is iffy, some know what to do, most don't. It will cost about 500.

Third option: new valve module. Cost 1000 to 1500.

Fourth option: transmission rebuild. Cost 1200 to 2000.

Fifth option: install remanufactured trransmission. Cost 2000 to 5000.

Be aware, driving it, and letting it shift hard will destroy the transfer case.

Tomorrow, mine goes into the shop for reman transmission and transfer case.

I had a GMC that did that. My auto had a vacuum shift module on it.
The rubber connection on the steel vacuum line was cracked.

I don't know if the newer ones are set up that way or not.
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

I'm afraid Ross's trans does not have that vacuum modulator that your
GMC had, Mike. Boy, those vacuum hoses leaking would really make 'em bang, huh? It would've been good for drag racing if gears would change at correct rpm's and tranny would stay together, tho' John's interpretation sounds like what we experienced with a dealership over the later years. IIRC, that 4L60E's shift pattern is "flashed" into the vehicle's computer ( since it has no chip to replace ) which controls each shift-solenoid--1-2, 2-3, and I believe 3-4th.
Wouldn't it be great if Ross could correct it with replacing a mal-functioning 1-2 shift solenoid!!! Sounds like other solenoids are working ok. HTH