Question about Transmisson and Rear End in 75 GMC Camper

[campingman]

Got a 75 GMC G35 Rally Camper Special dually with a TH400 and 4:10 rear. 69K Miles. Wondering if I should change the tranny filter and fluid or just leave it alone. It has a brown redish tint to the fluid. Works good and shifts fine. Just don't know if the last owner maintained it. Kinda scared to change the filter due to some trannys going out faster. I have the ability to suck out the fluid from pan to just do a fluid exhange. DEX III for this tranny right? The rear end is a 4:10 and was wondering which fluid I should put in it. Thanks guy for all your help on here. Great Forum!