I had similar problem with my 90 YJ with the 4.2. It turned out to be the engine misfiring caused by arcing within the distributor cap. I had a stock cap and rotor and all was fine as long as I was not putting any load on the engine. However when I tried to lug the engine or accelerate it strongly...lol..., it would buck until I down shifted. I did the Team Rush upgrade and it immediately made a huge difference. Just this mod alone eliminated all misfirings and problems lugging the engine at low rpm's. Do a google search and read how simple this mod is. I did mine with all salvage yard parts and probably spent a total of $20. Good luck in fixing yours.