Not much really. The 75 could be a troopy or cab chassis but when the 78/79 series came along in 1998,the 79 (cab chassis)got another 100 mm in frame length.
Because the troopy was too expensive to stretch(my assumption) they stuck with the old 75 series body and frame,but called it a 78 series.
The 75 was leaf sprung all round and the 78/79 got a coil front
The old 75 had a fold down windscreen on the cab chassis but the troopy was all one piece ,but the two bodies were interchangeable which they are no longer.
The 78/79 series also got the 1HD FTE engine(aussie only) ,same as 100 series but without intercooler
The newer model also went to 5 larger lugs instead of 6 lugs on the hubs.
According to Toyota this was to stop owners using the old wheels which would foul the steering on the coil front
Having used all the from 70 to 79,the new V8 model is still a 78/79 but with a 100 mm widened front to accommadate the wider V8.