I just purchased a BH92 for my MX5100 DT should the back hoe be able to lift the rear tires off the ground ?
It should if it's a normal duty backhoe. I can't speak for 3pt systems though as I've never used one. Most operators move the entire tractor around with the backhoe.
I put the front loader bucket down slightly curled up for stability but not to gather dirt and use the hoe to keep pushing the entire rig forward, lifting the stabilizers which have lifted the tires (slightly, I don't like the tires fully airborne).
I have seen comparison videos between excavators and backhoe's and laughed when the backhoe operator actually lifts the stabilizers, changes positions and drives the vehicle forward. He then changes positions again and drops the stabilizers and digs again. It's quite humorous as I've never seen a single backhoe operator do that on the job.
If your digger can't, then it's probably not built for 'normal' work and is just a landscape style digger, light duty, or it's not working right to its rating for some reason.