Long shot, but does it have an automatic transmission with a cooler built into the radiator?

Possible that cooler failed and is dumping trans fluid into the radiator, which would give you a similar oil sludge. Radiator replacement would be much easier than head gasket, although if you've gotten coolant in your transmission there could be issues there as well.

Good luck.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap