why does my 99 expedition blow hot air in the rear and cold air in front?

Your rear a/c line probably has a hole in it. Very common with vehicles of that age.

I had to replace the rear evaporator for about $400. When it went out, ALL A/C went out. All the freon leaked out. not sure how front unit can still be giving cold air.

Someone could have bypassed the rear lines? Twise, how long have you had your expedition?

I've had it for about 4yrs transam and it ended up being a blend door issue

Ahhh, yup that will do it to. That was where I was going to point you next if you didn't find the problem. Glad you got it fixed.

Yeah me too this cold Midwest was no fun last year without heat

Oh yea, heat would not be a good thing to be without. I live in he Midwest as well. God I hope this year has less snow!