My 08 g35 will only move 20 ft from cold start then loses all transmission functions. Just revs. When I switch to manuel mode it does nothing and the dash light stays on D when it should say 1. The reverse has slipped a few times and will jolt into gear when I rev itI’m guessing the valve body? Should I just throw whole new rebuilt transmission? The car has 150,000 miles, and have had zero issues with forward gears until this. Any suggestions would help a ton..

Have you checked the tranny fluid level? If the fluid level is good it's probably a smoked clutch pack. Used transmissions are cheaper than rebuilds, if the chassis already have 150k on it I'd just toss in a used tranny for half the cost since by the time the car has another 100k on it it's going to have significant issues across MANY aspects of the entire car so you probably don't "need" a rebuilt tranny that's good for 200k+ miles.

From what you described it sounds like it's just low fluid though. Is the transmission light on? Seems odd that you aren't throwing codes when it's in D and not outputting torque.