1997 jeep wrangler ac will not blow out of main vents,blow out at feet and defrost vents

Maybe your dashboard vents are clogged, obstructed or stuck in close position somehow.

If not, check the air hoses that carry the cooled air to the main vents are not clogged, bent, ripped or simply detached.
If its practical for you, check if they are correctly placed, clean and general conditions are ok if you can.

If above are not the issue, go to check outlets of the A/C unit, its filters for the possible reason.