I would test for spark and fuel this way
Remove intake's air tube(large tube to air cleaner)
Open throttle and spray starting fluid into intake.
Try to start engine
If it fires up and then dies you have a fuel problem.
If it doesn't fire then you have a spark problem.
The 50/50 test, easy, fast and tells you alot
(testing for spark by pulling a spark plug wire tells you squat, works on single cylinder lawn mowers, or minibikes, lol)
If it fires up and dies, put air tube back on and unplug TPS(throttle position sensor), it is a 3 wire sensor on upper intake connected to throttle shaft.
Try to start, if it starts then you have a short in TPS or it's wiring that is shutting off fuel injectors.
get voltage meter to test TPS, do NOT just replace it, sensor rarely fail, 90% of all sensor replaced by DIYers were good, save your money and buy a volt/ohm meter.
If it doesn't fire then spark is the issue, 1991 3.0l has TFI ignition, TFI module on the side of distributor needs to be tested, read here:
http://www.therangerstation.com/tech_library/TFI_Diagnostic.shtml