95 transport towing

I am towing a 2600lb trailer but its like pulling a billboard compaired to my 3000lb boat. I can easilly do 70mph with the boat but have to go 50 -55 to get the tcc to lock with this trailer & keep tranny temp below 180..
I know going slower is the obvious answer but is there a way I can get my tranny to lock up better? It slips out at the slightest hill or throttle change..
tires are 205 65 R15's . Should be 75's...would the shorter tire affect tranny this way? I would expect the opposite .....stumped. Is there a way to 'force' TCC lock? would a shif kit help? Tranny only has 50k on it.
Thanks,
Paul

[LMP]

Standard tires on this car are 205-70. 205-65 should only lower the load on the transmission.
I guess the boat has better aerodynamic characteristics than the trailer....On all towing operations I'd keep the tranny in the D range...allowing only third gear..and TCC can lock up on 3rd gear. I consider OD is not allowable for towing...either for the sake of OD gears or for the TCC.....specially given the very very low gear ratio. TCC is not built to handle high torque loads. ( i do not read your "cars owned info" in your profile....but I guess you have a 3800). BTW, what do you have to track tranny temperature?....
...mmm....You live in vineyard country don't you?....

Thanks LMP,
yes it is 3800. I put a mechanical guage in line with the tranny cooler lines closest to tranny output. I thought that would best represent the tranny temp....really gives me an idea if I am pushing too hard...best $10 I spent.