Odd thing with my 4EAT in my SH XT

perfectly normal.

The curb weight for the '99 was 3020 - 3120 pounds, depending on model.

The curb weight for the '10 automatic trans is 3,300 - 3,460 pounds, depending on model.

Essentially the '10 may weigh up to 430 pounds more not to mention that they were made 10 years apart. There are bound to have been a few changes.

My 99 rolls backwards in drive.

Slight difference in the idle speed would determine if it would roll back on a hill or not.

I thought the idle had somethin to do with it. Is there a way to have that speed changed so it doesnt roll with gravity?

On the old models there was a separate throttle position sensor and you could adjust this in certain cases when mounting it to vary the idle speed. Modern models have the tps as part of the electronic throttle system and are not serviceable separately.