Thats exactly what we did. Seems to work good now. I wish we had done that 2 years ago when we replaced the rad for this very reason. That rad was not cheap.

This may be too late, but I haven't been on for a while.
Go with the external cooler mounted behind the rad for the cheap fix, otherwise get the internal one repaired. High temps aren't going to cause oil to mix with the coolant, but they may cause the cooler to spring a leak if the pressure gets to high, although there should not be much pressure in the cooler anyway. It should be set in the return-to-tank line and effectivley open to atmosphere (in the tank) at the outlet side.

sorry, had head up *** syndrome. the heat issue still applies. check releif pressures and if there is heavy usage of any attachment you should look into a flow divider. toyota's hydraulic system far exceeds the gpm tolerance of most attachments and what is not used goes through the releif valve and HOT happens. check the temp of the oil in the tank. i have seen oil temps over 200 degrees and oil does not cool as readily as water does. if heat is the issue adding a second cooler / replacing the cooler with external one usually lowers air flow and leads to more problems with related systems. unless you a can find a cooler that fits somewhere other than the radiator area you may not gain much.

I am dealing with the hydraulic oil side. not the trans. This rad has 2 coolers on it. Thinking of rerouting the hyd oil into an auxilliary cooler.

extreme trans temp's. chech inch pedal op's. 2 stage inch system, 1st stage drops pressure by a little more than half and allows lots of slippage. oil gets VERY hot. use the **** out of it and shoot it with an ir thermometer.