You can drive your ATV on roads and trails that are already in use and not closed to motor vehicals.
There are few area that let you just drive off-road,and these area's are so crowde with the weekend dirt riders that it is hard to get sleep at night let alone a bear around.

Best bets for riding your 4-wheeler around and seeeing a bear would be the Belt Mountains,or the Linchon or Libby area,where logging is more prominent,and roads are more common.

I do spot and stalk cause baiting and hounds are not alowed in Montana,so you do get alot of excercise walking to vantage points to glass clear cuts and meadows.