+There are lots of kits for these steering boxes. It is extremely important you get the right kit. Yes, you can do it on the cab in situ, the oil needs to be drained, the bottom dog's leg control arm needs to be removed. You will need a large puller which you can probably rent. Make sure you mark the shaft and the arm so you know the arm and shaft are going together in exactly the right place when you build it back up.
Once the dog's leg is off remove the dirt cover and you will see the oil seal. A good pair of surclip pliers will remove the surclip and then you will have a metal and rubber seal to remove. Tap the metal seal on either side with a sharp metal punch, then with a small drill, drill through the metal seal. Screw in two small self tappers just enough that they are secure. Use two pairs of pliers and wiggle the seal out. There is another rubber seal underneath, pop that out and check to see if there is anything else. Make sure you note which way they come out so they go back correctly. (Top tip - take a photo on your phone as you start each stage). Clean the shaft with some fine wire wool and some clutch and brake cleaner. Before replacing the seals put a thin smear of ATF oil on the shaft, then rebuild.
Personally, I would take the box off and do the top, bottom and the power pump seals in comfort at a bench.
You will need a new bolt kit to put it back on to the chassis if you take it off.