Transmission slips out of 2 gears put in reverse goes in hard then pops out and wont go they have tried spider gears, sylenoid pack, computer, everything looks brand new inside transmission

Shift the clutch into first and second gear several times before attempting to shift into reverse if you have a manual transmission. Sometimes the clutch jams between the torque converter. Some minimal shifting between gears usually loosens the clutch, which enables you to put the car in reverse again.

It may just need servicing/adjusting.

Your reverse gear is actually first gear ... when you select reverse another 'cog' fits into the toothed wheels to change the direction of first gear ... to give you reverse...

It may be a sticky valve or just a bit of adjustment (workshop job..) that is required.

What you can try ..

Start the car ... move the lever through all the positions several times (foot on brake..). Keep your foot on the brake and then put it in reverse and rev the car. Hard!

Try doing it several times.

An auto transmission works on valves opening and closing. Reverse gear - in comparison with the forward gears - is seldom used. The reverse valve can 'stick'. Try forcing it to open ... frighten it ... but keep your foot on the brake...

Transmission is done. Nothing electronic will cause that, only mechanical

Servo or band worn out, o.ring gone hard or clutch pack burnt out.

One of the bands or clutch packs are burned out in the transmission. Or that piston could have a blown or ripped seal.
That vehicle has a 4L60E transmission I am fairly sure. If it is a 1 ton it may have the 4L80E. If you really want to know I could probably check my books and tell you exactly which clutch pack.
I should think you would have other symptoms though as well like first gear going straight to second or some such thing.
This is all assuming it is an automatic. If it is a manual stick transmission then you still have a transmission problem. Would need more information to track down what exactly. Things like noises and circumstances. for instance the shifter goes into the reverse spot but
nothing more then a funny noise. OR the shifter wont even go into the reverse position.

It sounds to me that the clutch material in the transmission might be breaking down. Before too long I would have the transmission rebuilt or replaced. If you wait too much longer the transmission will start to slip and pretty soon you will have NO gears but first and reverse.

do you know if you have a limited slip rear diff?
if you do your grease is worn out or if the fluid was replaced it need to have a friction modifier put in the diff (can be found at any auto part stores.)

if its not a limited slip diff you might have spider gears going bad. then you need to remove the cover on the diff and inspect your spider gears and these are the spider gears
and this is what a limited slip looks like
good luck
and dont forget to rate my answer
thanks
paul

The axle isn't seated all the way into the transmission.Inside the transmission is the differential,the axle goes into a side gear,called a spider gear,and slips into splines inside the spider gear.Once in to a certain point,there is a retainer clip on the end of the axle that pops out when seated all the way into the spider gear in the trans,and this holds,or retains the axle in the trans.If whoever worked on the car didn't have the axle fully seated to where the clip popped out,then the axle will eventually slide back out of the trans,disengaging the splines,and causing the vehicle to not move.Good luck.

I think you are saying the vehicle pulls forward,no matter what gear you put the shifter in?If this is the case,there is a problem with the forward clutch.I've seen this one over and over in many different transmissions and vehicles.The forward clutch pack is always applied whenever the vehicle is moving forward.It is never applied in reverse,or neutral.So,the forward clutch pack loses clearance from normal wear and tear,or a seal in the pack goes bad,and the clutches start slipping....and slipping........still slipping.....until they get SO hot,the clutch plates and steel plates fuse together,are welded together.This is why your Millenia is going forward in reverse and neutral.It's either that or you have a problem with the shifter cable,or linkage,and you didn't mention anything about the detents,or not being able to obtain Park,so i assume all that is ok