Scot:
a couple insites of things I see:
1. try to remove the slideing part and bevel the top & bottom edges of the 2.5" tubes so that it can't DIG IN the edges are nice and square which will catch and attempt to grab the upright tubes.
2. add wheels to the top and bottom sides. wheels on one side only will not help you very much. the next thing would be to use some chuncks of UHMW (plastic stuff very slipery and easy to work with) and mount them as blocks on top.
3. consider mounting the blocks not only inside & outside but front and back so all 4 sides are captured around the 2" square.
4. get the LIFTs center for weight distrubution even so that if you were to ballance the saw part on a pin it will stay flat & level on all 4 planes (left, right, front, back, and twisting motions should be as neutral as possable.) this will go a long way in making it help.
5. add some toggle clamps to HOLD it in place on each side after you are set in postion so it does not MOVE freely on its own. that is if it is not being HELD in place already but (you're design) doesn't yet appear to have it. must keep the blade from twisting up or down as the blade starts going through the tree! which it will do if the unit is not held ridgid correctly.
6. maybe think of one of the jack screw machines to raise/lower it.
http://www.surpluscenter.com has some that are 110 vac and some DC now for aobut 60 bucks with a lot of travel witch have 600~900 lbs of lifting price was 25 to 70 bucks + shipping.
I'll keep helping as needed. keep posting new pics as problems arrize.
Mark M
http://www.bright.net/~pfryman is my show where I work, built suff similar but I don't ahve any GOOD photos of them in the material handeling sections.