mountain towing w/ TM and 4.5L inline 6...

does anyone on the board have experience towing a TM in rocky mountains with a 4.5L inline 6 engine( a toyota land cruiser '96 model to be specific)? we fly fish, and have the toyota to get us to alpine lakes for cutthroat trout. of course, we would leave the TM lower, but it would still climb some Mt. passes. it's just that the toyota would be going further up.  does anyone have an opinion on how this engine would performin steep mountains? it has factory tow pkg, which includes oil and trans cooler. it is rated at 6500 lbs. but others say it is rated for 5000 #. i appreciate it!