My problem wound up being the Hydrostat pump. I took it to the local JD dealer and they tore it down and sure enough you could see how worn the pump was. I haven't received th total bill yet but the pump rebuild was to be around $2000 t0 $2500. The good news is the tractor is back to running.
My JD 4600 tractor had only 1,080 hours and apparently this sluggish symptom was very unusual. The pump tested fine. My John Deere dealer ended up replacing the hydro control and piston. The tractor had to be split and required special tools to do the work. The bill for parts and service was over $4,000. It works fine now.