Several questions need to be answered first. What size load? Did temperature go up when you went into reverse? While I've never had an issue with this on my truck or jeep, I can say that as a general rule, for sake of reducing wear and tear any time I'm able to use 4-low to back up my trailers, camper, or anything heavy regardless of tow vehicle I will. I also use this practice when I'm pulling the boat up the boat ramp.
As for your leak, I'd check your lines and hose clamps. Any time I've added trans coolers to my vehicles, I've always had to come back later to re-torque the clamps as the hoses will settle a bit. Hope this helps.
