I took my dad's 97 Accord( its an automatic) out to go to the bank and it felt like it was giving a lot of resistence whenever I changed the gears (from P -> R -> D4...). What the hell is wrong with it?

 

were you on an incline? auto trannys clunk when you shift on a hill or uneven surface.

 

If it happens in all the gears all the time, on level ground, then maybe the shift cable is full of crud. Lots of extra friction in the cable or maybe the cable is pinched or kinked somewhere?

 

Could changing gears while driving cause the problem? I don't drive it as if it was a manual, but I do occasionally downshift to 3rd when I have to merge on a uphill portion of the freeway.

 

Depends on what's really wrong. If the cable or the lever pivot is full of crud then it might mean someone spilled pepsi in there.

The cable doesn't know whether you're downshifting for an entrance ramp or putting it into park. If Honda really didn't want you to use D3, 2, & 1, they wouldn't have put them there.

If the real problem is internal to the transmission, then you have to know the exact problem before you can start guessing what caused it.