Tires: different tires front and rear, toyota 4runner, cheap tires

Question
I have a 4WD Toyota 4Runner that came with tires BF Goodrich Long Trail size  p225/75R15. One of the rear tires had to be replaced. I installed on the rear a pair of new cheap tires of the same size because I am planing to sell the vehicle in 2-3 months and will not do much driving until then. I then went to another shop for a brake job and the mechanics commented that the new tires are no good because they are of a smaller diameter. They were comparing the tires with the front tire off the vehicle, but when all tires are mounted and inflated to the same pressure there is barely a difference between front and rear maybe 5mm.
Are the new tires OK ?
Thank you
Talia

Answer
Talia,

No, the new tires are not OK.

The general rule is for all 4 tires on AWD's and $4X4's to be exactly the same - same size, make, model, and state of wear.  Do do otherwise is to risk a drivetrain failure.  

So you need to buy another pair of the same tires for the front.

And thank the guys who found this out by doing more business with them. and don't do business with the tire shop who sold you the pair - other than to get the second pair of tires.