5 at 125psi and 1 at 100psi is a 20% difference which is bad, you make have an intake valve that is not closing all the way, causing the back fire.
1989 2.9l will have a TFI system for spark.
This is a self contained system, it "talks with" computer but doesn't need the computer.
The distributor is like any other distributor system, it needs to be manually setup by you at 10deg Before TDC or it will be out of time.
Inside the distributor is a sensor in place of "points", the sensor tells the TFI module when the rotor is pointed to a spark plug wire on the cap, same as points did.
If this sensor was bad then you would have no spark so no backfire.
To correctly time the spark you need a timing light and to find the SPOUT connector, this is a Jumper that needs to be removed to prevent computer from advancing the timing while you are setting it;
Google: SPOUT connector image
To see what it looks like
It will be by the distributor on a 2.9l usually passenger side above intake manifold
Picture here: spout002.jpg Photo by gertsdsbs | Photobucket
You pull out Gray plastic piece to time spark, this disconnects TFI from computer, put it back after setting base timing.
First, double check firing order on cap, look here, about 1/2 way down page: Ford Ranger/Bronco II 2.9 liter Engines
Then remove distributor cap so you can see the rotor
Mark #1 wire placement on distributor so you know where that is for rotor position
Rotate crank manually untill rotor is close to #1, then put crank TDC mark at 10deg Before TDC, look at rotor, if it is not pointed at the mark then loosen distributor and adjust it so it is.
Snug up distributor
Put cap back on
Hook up timing light
Time to 10 deg Before TDC, some like 12 deg better, your choice