NO power when pulling trailer

Folks,

Got a 96 2.4l K24 w/160k on the odo. i've put 100k on her and did the little minor maint that has come along (oil, filters, distributer cap/rotor, plugs, wires, clutch, batts, MAF (not minor in $$) EGR valve cleaning, Seafoam every now and then through the brake booster and the tank) and really have no problems. she gets 23-25 Hwy mpg consistently. Strong engine, I can put anything I want in the bed and she rolls right along (w/the front end up of course).

I put something behind her and she's a dog.

For example, yesterday I drove about an hour away to buy a 6'x10' utility trailer w/wood floor. Mesh floor trailer specs in at 700#s so I'm guessing with wood your talking 800#s tops for this one empty.

I pulled the trailer home EMPTY and barely was making it up slight hills, at times having to downshift to third and this was on I-20.

Is this normal? I'd think not, but have nothing for comparison so am looking for your experiences.

I'm going to go ahead and replace the o2 sensors and check for vacuum leaks under the hoood.

Anything else?

Thanks -