heater vent will not readjust to any position
Most likely your heater core is leaking.
The HeaterTreater is an after market kit that solves heater and AC blend door malfunctions for the Jeep Grand Cherokee and Laredo. It is much cheaper than the dealership fix and is easy to install. For diagnosis and info, check out www.heatertreater.net or search eBay for "heatertreater".
It is probably your thermostat. That will make cold air come in. It's probably stuck open. The part is only about $4.00 at Auto zone.
There are several things that can cause heater problems in your 1998 Jeep Grain Cherokee. The most common cause of heater problems is low fluid in your cooling system.
About $600
Yes
Do you live in an area that you need your heater?
check the fuse or the ground on the heater
The heater core is inside the hvac housing behind the dash.
blend door failure is a common problem on 97+ Jeep Cherokee
No, it is not required. You can remove it and splice the heater hoses together.
The heater core is inside the hvac housing under the dash.