Takes a long time to heat my 1998 yukon

Low on refrigerant , A/C compressor , rear expansion valve . You best bet is take it to a qualified repair shop

Sounds like the cooling system is low and has air pockets. The rear system problem could be something electrical.

Rear heat not working ? The blower motor isn't working ? No air coming from the rear vent's ? Is it just a yukon or XL or denali ? What size engine ? 4.8 , 5.3 or 6.0 l Manual or automatic climate control ? Does the blower motor work on any speed ? The front an rear heater system are seperate , have there own electrical circuits . Could have a couple of different systems ,could have a control processor or relay's an a resistor block . Do you know how to test automotive electrical circuit's using a wiring diagram and a DVOM - digital volt ohm meter ? Plus to get at the rear heater-A/C unit you would need to take the right rear trim panel off .
2003 Tahoe No Rear Heat Fix Suburban Yukon XL Repair

The cluster is powered by the radio fuse. Is the radio working?

1) Check your water pump is working OK and
2) You may have an airlock in the system. Take the expansion or header tank cap off, set your heater control for hot and exterior air. Start the car and run it till warm. With luck you'll run any air locks out of the system. While warming up, and being very, very careful, try squeezing the main radiator in and out tubes (The fattest) this may also move any air locks on. BUT BE CAREFUL, STAY AWAY FROM AND FANS AND PULLEYS. IF NOT SURE, DON'T DO IT OR STOP THE ENGINE FIRST.

With luck, you should soon start to feel warm air coming from the vents.

you need to check the blend door motor. it may be getting stuck halfway so its letting the cold air mix with the hot air thus making it not as hot. Its a common problem on these GMC trucks and SUVS.

check blend door actuator behind glove compartment it keeps your compressor from coming on.check to see if the compressor is coming on when you turn the air on.

V8 engine? That would be drivers side front. Remember that TDC is number one piston up (On the compression stroke, not exhaust stroke), timing marks at zero.

iwould check the selector motor,and also replace the thermostant,start with the stant first,the selector motor is what runs the door for hot or cold the motor is a common problem,if the temp gauge is showing a quick warm up temp then its the selector motor, if it takes a while to reach operating temp then replace the t-stant,also make sure the coolant is full in radiator,check when cold or warm,dont open when hot, hope this is helpful.

most likely problem is that the cooling system has air trapped in it, if you can find someone with an AIRLIFT tool to bleed the system