What are your rpm's like? If you're having problems getting any high rpm's, then you could have a plugged catalytic converter. My 89' yj w/4.2 had absolutely no power and was really a dog. I couldn't even get 2800 rpm's when I first noticed I was having problems. By the time I figured out what was going on, all I could get was 1200 rpm's and I couldn't even climb a slight hill going down a paved road.
89' YJ - 4.2 - NV4500 - NP231w/sye - 1" Body Lift - 2 1/2" Rancho Suspension Lift - 31" Firestone MTR - AR Outlaw II - Tomken Bumpers F & R - Warn 8000 front - Warn 6000 rec. mount rear - FlowFit Seats - Tom Woods Rear & Custom 2" front driveshafts