07 P71 Slipping

Ok so long story short I purchased a 07 p71 at an auction for 500 bucks. When I picked it up it didn\\'t have an air intake, battery, or headlights sockets but the car fired right up. Reverse worked perfectly but forward would slip I was assuming because it detected no air intake so I limped her home where we installed an intake I picked up at a junkyard and put a new battery and replaced transmission fluid and filter and no there were no metal shavings in the pan . So with overdrive off I only had to rev the car up to 3500 rpm one time in order for drive to engage then it would stay engaged until I took it out of drive . It\\'s also worth noting that the car makes a whine on takeoff and only in 1st gear . So I drove the car that way for about a month when about a week ago it would slightly vibrate after engaging drive and yesterday the transmission finally gave out and is in a limp mode (wrench light) and Everytime I give it gas it will just rev up and move slowly . Also ever since I got the car it makes a pretty loud tick kind of like noisy lifters but this sounds more in the transmission and it ticks at the same rpm as the engine. The only codes I have came across was p0720 any advice?

At this point you are pretty much guaranteed to need a tranny service of some sort. If the clutch pack is not worn you may just need servos but my guess is you probably need a full on rebuild. When my 99 was slipping like that I finally took it in and sure enough the clutch pack was shot and so were the servos to change gears. The shudder you mention in was in my case a failing torque converter a common issue for the stock converter.