Im gonna start hauling a medium gooseneck horse trailer with two horses. All together im estimatung about 5,000lbs, with my 97' automatic 5.4L 4x4. Im adding a brake controller, heavy duty shocks, helper springs, heavy duty leaf springs (4 leafs instead of the stock 3), possibley a trans cooler and a shift kit. I also have 265/75/16 cooper s/t's instead of the stock 255/70/16's. How do you thing my truck will handle the load?

 

If you are truely only towing 5k, that would also put your tongue weight at only 1k. You should be fine.

But....
An aluminum skin or aluminum 2 horse GN trailer with a zero foot short wall front tack weighs about 3200lbs empty. Since 2 horse GN are not very common I'm gonna assume it is a 2' short wall and a 3 horse trailer will put the weight up to 4000lbs empty.(that's why better built GN trailers will have 2-5.2,6 or 7k axles... usually only the 2-3 horse bumper pulls will only have 3.5k axles) Next add 2 horse and tack and all the stuff horse people normally haul with and you will be waaay over that 5k estimate....closer to 7k. Tounge weight now goes up to 1400lbs, add the weight of the hitch (250#'s) and your truck load is now 1850lbs.

Could the truck take a 1850# load?
Yes, but you are now beyond your Rear Gross Axle Weight capacity.

Do people do this?
Yep... but in any bed mount hitch you are better off with a heavier duty truck like a F250,350.

Last thing to consider.
Look at the 1997 truck frame directly above the rear axle. It is the weakest point. A Toyota Tacoma has a stronger looking frame than the '97-'03 trucks and most wounld not consider a Tacoma adequate for the load you are referring to.

Now all the guys that do it are going to chime in.........