Hard shift? Or Thump at differ. Acceleration points

I have chevy Blazer 2002 Zr2 4 wheel drive. It Started today, I accelerated on the fwy to 65 and it seemed to have a bit of thump. I let off the gas, and stayed at a steady 60 for about 5 mins and I slowly accelerated again. Didn't do it consistently but would do it occasionally throught my drive on the fwy sometimes at lower speeds. It seemed to be give me a harder thump each time it did happen sometimes at 30 sometimes at 45 sometimes at 65. It seems to be going into gear but happens after the fact, my oil gauge started to fluctuate in the last 10 mins of the drive. Not to much else was changing as far as gauges.. Last time pumps and hoses needed fixing, and once something was loose on the engine I think it was pistons maybe? I haven't driven my car since. Going to check all the fluids in the a.M. Any idea. Could it be the transmission??
Thanks what should I check for if I drive it again?