'09 Transmission/CC question

[MarkW65]

I just bought a used '09 Sierra. 5.3 with automatic transmission. When stopped on a hill, the truck wants to roll back when I take my foot off the brake to prepare to accelerate. I have to really get the RPMs up to get it moving. Also, I stopped on a downward slope this morning and was trying to back into a parking spot and it did the same thing in reverse. It drifted forward before I could get it to back up.
What's the problem, the fix, and the cost?
Thanx so much. I really like the truck and hope this can be taken care of.

 

[98trck]

how many mikes on the truck?
when was the last time the trans was serviced?

 

[MarkW65]

I just bought the truck last Thursday so not sure when it may have been serviced last.
It just hit 70,000 miles.
UPDATE
I just checked the CARFAX they gave me and it does not show any transmission service though the manual says it should have a fluid and filter change at 50,000 miles (severe service).

 

[MarkW65]

I noticed today that it doesn't do it every time but that only makes it scarier when it does.

Anyone?