91 Tracker stumbles above 2800 RPM

[fixkick]

Checked all the items you suggested and all were good. Disconnected O2 sensor and problem disappeard but then came back (but not so pronounced) in about 20 miles. Figured the ECU took a while to adapt to disconnected O2 senser. Installed new senser and problem gone, thanks for the help
Steve

that is one good test 02 pulls put the 02 voltage to 0.45v.  ( 0 to 1v is range)
0v = lean, AFR, ECU goes rich max.
1v = rich , AFR ECU goes lean

stuck at 0 or 1volt both cause ECU to cause TRIM to get stuck  full lean or full rich. on the rich side it misfire, and spark plugs go black tipped
the lean side , hesitations happen at throttle TIP IN, with toe, and then correct. (mostly)

02 bad or mainfold gasket leak down low can make 02 stick at 0v
some O2 the heater inside (rare) most burn out but some leak heater current to the sensor side (CELL+) and cause AFR to go SUPER LEAN. that is one nasty 02 if true.
02 are engineered to fail at 0v, this is their dream and is mostly true. but not always.

try new O2. cross fingers just that.
on the other had removing an O2 can hide other problems. (masking real) low fuel pressure can be hidden by removing (connector) O2. as 2 wrongs make a right (nearly)