My 1999 sierra 4x4 has a vibration at very slow speed (it almost feels like the tires are square)When I accelerate it seems to go away. It does also shudder when coming to a stop. Twice last week when pulling away from a stop sign I heard a loud bang come from somewhere in the drivetrain.the whole truck felt like it jumped.

SOURCE: 1991 GMC SIERRA Shaking in the drivetrain

crankshaft balacer going bad,can cause the crank to break if left unchecked

SOURCE: 2005 GMC Sierra 1500 rear noise

you need to check you universal joints usualy the one on the rear axel is the first to go. i recomend to put bolth in at the same time. the new ones will be greasable so dont forget to give them a little every oil change

SOURCE: Vibration

your highway acceleration problem could be a coil problem. Not sure if that year of chev has coil packs or not but experienced same thing with ford 5.4L. went through 3 coil packs and did the same thing each time before biting the dust. Transmission seems to be sticking on the 1to2 upshift and could be a sticking solenoid in the valve body of the tranny. If the shifting problem persists then the tranny deffinately has to be rebuilt. It's made it quite far if tha is the original tranny, you cant expect too much life out of it if it has been worked hard. I highly doubt a fluid change would help at all. Or if you want, you can buy a can of new truck from GM for about 25 to 35 thousand bucks! Typically works great!

SOURCE: 1998 GMC Jimmy...4x4 / transmission issues.

Lets take this piece by piece.
The 4x4 switch is defective.
A driveline vibration regardless of engine speed is usually a shaft or cv joint. To verify a visual inspection is necessary.
The poor acceleration in any gear is the begining of torque converter problems( stator or pump).As long as ATF level is correct.
I recommend having a transmission repair shop look this over and preform a pressure test of the main line of the transmission.(it may not be necessary if the condition of ATF is burnt). Good luck

SOURCE: i have a 2000 gc

hello daab....get it up in a lift and check both the front and the rear drive shaft universal joints. one of the is most likely worn and wobbly. Joe