Could very well be the starter, but a test would be: Do you have good voltage between the power cable at the starter stud and starter case when the key is in the Start position?
When you described some of the supply/ground voltage checks you did, I couldn’t tell for sure if you measured those voltages with the starter circuit energized (i.e., the power/ground circuit fully loaded). IOW, for example, did you get full system voltage downstream of ( < very important word there) the jump posts with starter circuit energized ( < also important word)?
Again - I wouldn’t be at all surprised if Dan is right about the starter being bad. But a check like I suggested should answer the question of if the problem is in the starter itself or “somewhere else”? You may have in effect already done the test I describe, but, again, I’m having trouble determining from your post if you did it “proper”, i.e., looked for voltage drops in power supply and ground with starter load applied (circuit energized). My apologies if you stated that, but if so, I’m not seeing it.
Common high resistance points if low voltage downstream of jump posts with load applied would be 1) at battery post connections (loose and/or corroded), and at jump post (either pos. or neg.) connections (usually loose, with possible resulting heat/arc damage).
I know you said you jumped the Jeep battery into it, but not clear where the clamp points were (for example, if upstream of the jump posts on the car, you have not ruled out a weak/high resistance jump post connection).
'98 LXi - Later Concorde gages (black w/ chrome rings)/'99 LX - LHS gages (white) - HIR bulbs