1991 Jeep Wrangler bogging down at 2500RPM

Hello all, New to forums but not to Jeeps. Recently talked the wife into letting me buy a 91 YJ. We love it. 6cyl. Automatic. Heres the rub: When revving it up, she starts bucking and shaking at 2500RPM, even with the throttle floored she will not go any higher. The 4wd light was on, but it was in 2wd. Now the light seems to go on and off when trying to drive it. She was shifting hard at first and now it seems like theres just no power getting to it, it wont go fast at all and has trouble on the slightest road hill. Flooring it and it will still only get to about 25mph. I've changed the oil and filter, tranny filter and fluid, spark plugs and fuel filter and still the same problem. Any ideas, thanks so much for your Help/Time.