Model 870 Diesel in Crankcase

I have a JD 870 that I purchased about 1990 that has around 550 hours on it. When I changed the oil last fall I noticed that the level was higher than normal and the drained oil seemed quite thin. Since the oil change I've used it for another 10 to 20 hours and I noticed that the oil level is again rising.

I'm pretty sure that diesel in leaking into the crankcase somewhere. I suspect that the culprit might be a bad seal on the injection pump but I'd like to avoid pulling it and sending it out for service if the problem is actually elsewhere. I'd like to hear from others that have had the same issue on this model as to what the issue was. Also, if anyone has some suggestions for things to check that would be great, too.