From the parts/service literature from their website

-Work Platform Drives In Slow Speed Only-
probable cause:Remedy:

1. Speed switch (S7-1) defective.
1. Replace switch.
2A. Differential valve coil (2H-18A) or speed valve coils (3H-18A-1, 3H-
18A-2) open or shorted. (Models 3220, 4620 and 4626)
2A. Check continuity. Replace if defective.
2B. Pilot valve coil (3H-18A) open or shorted. (Models 6826 and 6832)
2B. Check continuity. Replace if defective.
3. Dump valve coil (2H-21) open or shorted.
3. Check continuity. Replace if defective.
4. Defective wire #18 in arm cord assembly or loose connection at pin #18 on terminal strip.
4. Check continuity. Replace if defective. Tighten connection.
5. High Speed Limit Switch (LS1) open or shorted.
5. Replace switch.
6. Diode (D17-2) open or shorted.
6. Check continuity. Replace if defective.
7. Defective wire #18, #18A or #21.
7. Check continuity. Replace if defective.

You can go to skyjacks website and to their "manuals" section and download it to help you with location of components etc.
"https://techpub.skyjack.com/en-au/document/?type=manual&id=1463"
If this link does not work then you can go to the website and at the top there will be a menu choice at the top of the right side of the page labeled "MySkyjack" you can find where to go in there.