WILL A 255/70/70 TIRE AND WHEEL FIT A 2004 JEEP GRAND CHEROKEE?

Either of those is possible, but also possible your mechanic is wrong. Your searching warrants further checking. Did he actually look at your old wheel hubs on the jeep? Find the abs sensor on your jeep, and then you will know. I think you may be right, and your mechanic is wrong.

ITS A 95, and old. and has spring sag.
find a tire /wheel shop and let them do the magic
on a real car.. take it to them. let them fit it.
are you going to guess at $1200 a try?
trial and error?
get them to buy into a fit. at 1 price, dig?

Check your ball joints. also, if the tires are somewhat worn many times they won't have enough rotating mass to overcome side harmonics and can shake but most oftern the joints are worn. No play is acceptable there.

The lug pattern for a blazer is not the same even if tire is the same size. Afew wheel spaecialist can repair you wheel though

Check the transmition fluid level if you know how. The Jeep should be looked at by a mechanic as soon as you can do it. Check for fluid on the undercarriage that may be from a transmition leak.

From what I know about that system I don't think you can without possibly damaging the transfer case and, I don't think you really need to. The way the system works, the front wheels are not really engaged unless the transfer case "sees" slippage or a dis-similar turning speed between front and rear wheels (why it's important to keep all tires exactly the same in size, brand and tread depth) when it sees a difference a coupler inside the case heats up and makes the engagement. On dry ground, there is no slippage and the unit remains in two wheel drive.
Therefore, I'd leave it alone.

Stock Tire - 225/75R16 >Search Tires Section Width: 8.85 in 225 mm Rim Diameter: 16 in 406.4 mm Rim Width Range: 6 - 7 in Overall Diameter: 29.28 in 743.71 mm Sidewall Height: 6.64 in 168.65 mm Radius: 14.64 in 371.85 mm Circumference: 91.98 in 2336.2 mm Revs per Mile: 710.3info from 1010tires.com

go to tirerack.com and you can view any wheel and tire on your jeep that will fit

Front and Rear Tires are P225/75R16

Also check this info in WJ Jeeps (1999-2004 factory wheels and tires)

Good luck!!!

under the back seats, jack is under the driver's side backseat, tire iron & rods are under passenger's side backseat.