I'm no genius, but our Aussie Friend
First things first.. Like all would say? How fresh is your Transmission Fluid? Up to the level? Recent filter?
Do you have reverse? Now, pressure is everything in a transmission, no cooler to radiator cross contamination? this will be a slippage on all gears. The main oil pump on the transmission could be damaged too.
Download the master.pdf document. On page 7 the troubleshooting section will outline transmission component problems. The document is good for 722.3-722.4 & 722.5 Transmissions. Study the master document to familiarize your self.
See post 26:
Now, before you go nuts chasing B2 problem or the "metal pins" problems. Check the vacuum lines going to the transmission, and test them for vacuum with your finger while you accelerate the engine. Next, disconnect the bowden cable at the throttle-It is a ball-joint-you can use a screw driver-It is easier with a fork like tool-A plastic door panel spoon works the easiest. Check the bowden cable for it's travel(1 inch travel) and it has a springy feel. The springy feel is important as this bowden cable goes to the vacuum modulator which actuates a active piston on the valve body.
If the Bowden Cable and Vacuum lines check out okay.
At the back of the transmission is a black round connector(about 1inch in diameter) make sure it is plugged in. Underneath the transmission filter are three(on the 722.5) wires(green, black and white) terminate make sure the Philips screws are tight.
Lastly, you can remove the valve body piston plates for a broken spring. The B2 Piston can be removed from the outside(it is the four inch round plate) on the right side of the rear of the transmission. The link below on the 15th photo shows the black round seal that hardens-up(on 722.5 behind the black seal is a metal sleeve in the photo-722.5 has a plastic one-it will look yellow). The piston shaft should be smooth with miniscule small smooth waves(no sharp ridges or large waves).
How to Replace Your Oil Cooler Lines
I personally, do not think your problem is inside the gear train or clutches(but I'm not saying the clutches are worn) Your problem may reside on the B1, B2 and valve body. If you are a DIY'er, you can inspect the problems. You do not DIY! Visit a transmission shop.