I gave my '91 Suburban a tuneup (new plugs, rotor, cap) in preparation of towing. The car seems to run fine, but when I hooked up my trailer with a 4,000 pound load, the engin started to miss while going up hill. Down hill and level was no problem. Any ideas on what the problem is? Did I screw up on something? Thanks

I would check plug gap and fit of the wires onto the plugs...
it is also possible that you may have cracked a plug while installing or have a bad plug wire.
Having the truck put on a scope and checking the secondary ignition output will tell the true story.