Is the leak coming from the front? If so, you have a front pump seal that is leaking, and the transmission will need to be removed but with the 3rd and 4th gear slipping I would have the unit rebuilt. here is a video that shows the steps on how to remove the transmission on a similar vehicle with instruction for your truck in the images below.
https://youtu.be/3vJAojsrfuU
1. Disconnect battery ground cable.
2. Remove air cleaner assembly.
3. Disconnect TV cable at its upper end.
4. Raise and support vehicle.
5. Remove propeller shaft from vehicle.
6. Disconnect speedometer cable, shift linkage assembly and electrical connectors from transmission.
7. Remove transmission support brace attaching bolts from converter cover, if equipped.
8. Remove converter cover, then mark flywheel and torque converter for installation.
9. Remove exhaust crossover pipe and catalytic converter attaching bolts from vehicle.
10. Remove torque converter to flywheel bolts and/or nuts.
11. Disconnect catalytic converter support bracket.
12. Position a suitable jack under transmission and raise slightly.
13. Remove transmission crossmember to transmission mount bolt.
14. Remove transmission crossmember to frame bolts and/or insulators, if equipped.
15. Slide crossmember rearward and remove from vehicle.
16. Lower transmission.
17. Disconnect and cap transmission fluid cooler lines.
18. Disconnect TV cable from transmission.
19. Position a suitable jack under engine, then remove transmission to engine attaching bolts.
20. Carefully separate transmission from engine. Install torque converter holding tool No. J-21366.
21. Remove transmission from vehicle.
22. To install transmission, reverse removal procedure.
Check out the images (below). Please let us know if you need anything else to get the problem fixed.
Images (Click to make bigger)
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Monday, January 2nd, 2023 AT 12:11 PM