Had the same problem with my 2004 XT chain tensioner.
I have also risen the rear up as you and had the same problems.

Make sure that when you replace the tensioner+screw that the white/yellow roll moves freely with no obstructions and that the inner tube is oiled.
Also make sure the chain is not set too tight and you replace the inner white plastic tube in the right way.

One other thing regarding rear rising in combination with a much larger rear sprocket:
Due to the 48 sprocket i have fitted and the resulting greater amount of torque on the tensioner by doing wheelies, the tensioner on my bike has broken off completely only to leave a little metal edge at the place where it used to be.

still have to make an appointment to have a new one welded on. So be ware, it's not that strong...