is it a straight swap to put td5 axles onto a 300 tdi have range rover front axle from the 80s ? back one ?

Im not sure on the td5 axles guessing they will fit, but Im sure someone will be along shortly to confirm/deny

Defender td5 yes Disco td5 no, Disco do not have an A-frame in td5

As above def will but the disco ones have hockey sticks like the front def axle.

The other thing to check is the width of the front radius arms where they go into the brackets on the axle. Some of them (around the 300tdi vintage I believe) had narrower ones. Axles with wide brackets can fit the narrower arms, if spaced with M16 washers. If it's a narrow bracket axle, you might need some narrow arms to fit it. The majority of arms and axles are 'wide' it's just a few that aren't.

Yes, otherwise on the Defender the 300tdi (and even earlier) the set up was pretty much the same as on the TD5. The Disco 2 had a different setup.