Fuel Problems

Hey, I've recently been having what seem to be fuel problems. My '89 Laredo started bucking really bad upon acceleration and wouldn't stay running. Because of that, I replaced the fuel pump which fixed this problem for about 2 weeks. Now it idle's fine, but upon acceleration It feels just like the brakes are applied upon acceleration, right around 2000 rpms. I've thought it might just be a bad batch of gas (King Soopers 85) so I put 5 Gallons of 91 octane shell gas, and a bottle of Heet. The Heet helped a little bit, somewhat noticably so I poured another bottle in. The second bottle hasn't helped at all. I did replace the fuel filter along with the pump. I'm at a loss of what could be causing this. Any help is greatly appreciated.

Thanks!
-Eric

P.S. It's a 4.0L 4x4 MPI

have you actually checked the fuel pressure?
the regulator could be bad.
It could also be a timing/ignition problem cause by a loose timing chain or something else suitably weird.
Just for fun here's what I would do.
Check fuel pressure.
check the TPS and MAP sensor voltages.
put a timing light on it.
just a thought, if your xj has one of those ballast resistors on the drivers side fender I would bypass that too.

An O2 sensor change fixed my bucking problem (88 XJ) at around 2000 RPM. Seemed to be worse while trying to accelerate, during lower vacuum operation, throttle mostly open. I also noticed the motor pretty much crapped out above 3600 RPM and I really couldn't red line the motor. The O2 sensor controls most of the fuel delivery, air to fuel ratio, and can run your motor too lean under certain circumstances.
On my 87 the front harness (from the front end of the injector harness down the front to the O2 sensor, engine temp. sensor and knock sensor) was cooking on the exhaust manifold and would do almost the same thing. The O2 sensor wires melted together.
Bad gas has made mine hard to start and idle pretty rough, though it usually smoothed out some at higher RPM's and when the motor was warmed up.
A fuel pressure gage is a handy thing to have, it helps eliminate fuel pressure as the problem when troubleshooting. Most times when a fuel pump is getting tired (or the filter is plugged), in most vehicles, the first thing you notice is it feels fuel starved at higher speeds on the interstate. The fuel pump, just doesn't put out enough volumn, though it may have good pressure at idle or lower RPM,s.
I went through almost the same process you did, first checked fuel delivery,pressure and volumn, then timing (distributor index), on a hunch I swapped out the O2 sensor which curred my bucking, at around 2000 RPM, problem.