Boat trailer fishtailing

I have a 1995 19ft Ranger Walleye boat with a 150 hp. Merc on the back which is mated with a single axle ranger trailer.  I've never had trouble trailer the boat until this year, and I'm wondering since I added a 9.9  kicker motor on the port side if that could result in the trailer swaying back and forth?  The other day I was on the highway doing about 60 mph and the trailer started FISHTAILING severly without reason, which scared the living crap out of me.
Other details of significance:  Total weight of boat and trailer is approximetly 2800 lbs.  The big Merc 150 weighs around 500 lbs, and the 9.9 Merc bigfoot weights about 130 lbs.  My tow vehicle is a 2005 Nissan Exterra (v-6) with a gross vehicle weight around 4200 lbs.  This vehicle does have a relatively shortwheel base, which might influence this swaying.
I've researched the problem a little bit and I am going to check the following:
-The air pressure in the tires (15'' tires on there currrently say max load at 50 psi)
-The height of the trailer ball
-Determine the proper weight distribution for the boat.  I've been told having too much weight on the transom and not enough tongue wieght could cause this "fishtailing" to happen.
-The boat is already positioned as far forward on the trailer as it will allow.   
I would appreciate any help and or tips anyone out there can offer, or have experienced a similar problem firsthand.  Thanks. 
 

Check brakes and bearings but it probably is a light tongue problem.

I did trailer the boat for about 1 year without the kicker motor and never experienced any such "fishtailing" problem. 

My 115# Honda kicker makes a big difference when it's strapped on the back of the boat.  I only put it on when I'm going to limited lakes where I have to run it.

I would almost gaurentee it is the light tongue wt problem. I had a 19 ft century  I/O , 302 ford engine with a 30 gallon gas tank mounted in the front of the closed bow boat . Any time the tank was below half i experienced a severe swaying problem , it swayed the least with a full tank , as others have said the 9.9 threw the tongue wt off.

20% tongue weight will cure the problem

I had a simmilar problem with my boat that I have owned for many years. I could not figure why my transom straps kept getting shorter and harder to connect and my tounge weight was getting lighter. Here what was happening was the front roller with the hand crank was slowly moving back over the years. It must of slipped back 5 inches. I just loosened the u bolts up and moved them foreward to where they were originally. The boat seems to ride great now.  Another thing is is your toung parralell with the ground? I have an Xterra and it rides high.(NEVER TOWED WITH IT)Your ball hitch might be riding to high try a drop hitch to lower the toung. Maybe your Xterra needs a sway bar in it for trailering?

Thanks for the link Beev.
 
They must not bee too keen on enforcing it.  I've passed and been passed by many troopers on 79 @ 65mph and have never been stopped.  There's always a first time though.

ORIGINAL: beeverfishing

Just a reminder, Pennsylvania has a Maximum Speed Limit of 55 MPH while towing a boat. 
 
http://www.boatus.com/trailerclub/matlantic_laws.asp#pa

Where is that in the PA code?

A quick fix for the tongue problem could be a spoiler wing at the front of the boat! Might be easier to install and remove a spoiler than the 9.9.
Although this will not help with gas consumption!
Jeff