Wouldn't it take longer than 3-4 years to develop and market a 4wd with a new frame design? We're talking 1985-6ish to the 60 series being introduced in 1989? I have no experience with manufacturing or engineering. I realize the cab is more of an update, the engines and trannys are carryover (or a drop in of the Cummins in the 8960). I don't know what they used for axles, but I'm assuming the front axle on a 60 series is a new design, given the 30-40-50s had a pivoting axle? It would be interesting to talk to a Deere engineer about the development of the 60 series.
Thinking further, I went to the Farm Progress Show in fall of 1988, and I'm pretty positive they were demoing 60 series and not 50 series 4x4s. And I think the Versatiles were a mix of red/yellow/black and blue/black. I've got pictures socked away somewhere that I took that day.