There are so many things it could be.
The line from the throttle body to MAP sensor could be bad. Intake manifold bolts could be loose. You're gonna have to check things out.
Also would be a great idea to complete Tips 1 through 5 at www.cruiser54.com soon.
Dana 35 in the rear like many XJs. Dana 30 in front but in an IFS configuration.
I have all the engineer's hand written notes and part numbers for putting an AMC V8 into an Eagle......
well, here's my experience. 81 Eagle Wagon limited. Had high gears like yours. Put 3.54s in it. Little bigger than stock tire. Towed a heavy tent trailer from Arizona to Colorado and back. Worked great. Also Prescott to Tucson and back.