Clydesdales/Athenas (200+ lb / 91+ kg) - Higher Handlebar Help--Stems or what?

So my bike and I are bonding, but my lower back isn't liking it. I'm riding to get back into shape. Given my location my daily ride is on roads, but I do like to get dirty on the weekends and hit trails.

I've adjusted the seat so that I'm able to reach the pedals properly, but now I think I need to raise the handlebars as my lower back is aching after even short rides.

With 1.5 inches worth of spacers under my stem, the bike shop says my steerer is as high as it will go and is suggesting either an extender or adjustable stem. I've known people who have broken these and they were much smaller than I am (340 lbs 6'4"), so I'm not too crazy about these options.

Does anyone have an idea on whether replacing the bars or stem would be a wiser idea? I'm fairly mechanically inclined, but I'm curious about the difficulty level in replacing the stem.

I ride a Trek 4300. Outside of the saddle, all parts are currently stock. Ideally, I'm looking to raise the bars another inch. The current reach to the handlebar is fine, but if its a little less I won't mind that either.

What is your current stem? There are stems out there (30 degrees, for example) that might easily raise your handlebars 1".

I switched from my Worksman cruiser to a new bike with lower handlebars. Result: A small amount of back pain after riding. However, what I discovered was that it also went away by itself after a few days. I guess my back is being stretched a little more than it's used to, and some getting used to it was all that was needed for me. Not saying this is the case with you, but something to be aware of. I'm not a big fan of stretching exercises, but this is one case where it might help.

What is your current stem? There are stems out there (30 degrees, for example) that might easily raise your handlebars 1".

Its the stock Bontranger stem that came on the bike. 25 degrees 120mm

I stretch, however, I should also add that I have somewhat of a limit to my mobility. I have 3 fused vertebrae due to an auto accident. I thought the soreness would go away, but after 6 months of consistently riding 50 miles a week, its not.

A steering tube extender works just fine for road riding, but I have no idea how durable they'd be for mountain biking.

They go on an off with a couple of allen bolts, so you could try it out for road riding, and then take it off for the trails.

For $15 or so, it's a pretty easy solution.

High rise cruiser bars for this man!
http://store.comcycle-usa.com/Enlarge.aspx?id=932276

They look funky but I've got steering tube extenders on two bikes.
They work well and as said before they go on and off quickly.

You're probably going to hate this answer, but in my experience strengthening your core and a bit more stretching are probably the best solutions. Many of the leg and hip muscles attach in the lower torso. Add to that the abdomen weight pulling on your spine and it's pretty easy to see why you have some pain.

I have a steerer tube extender and I love it. it seems rock solid. who told you someone broke one?

mine is 5" tall and brought the stem up 3"

I'll bet you would be fine with more than a 1" lift of the bars. 1" isn't much at all.

on my old-school steel 10-speeds I always install taller quill style stems

I just ordered a stem extender for my Cannondale CAAD9. I was maxed out on spacers and thought a couple of inches would help me with the longer rides (my back and shoulders get worn out before anything else). It will probably also make riding in traffic a little easier. It should arrive in a couple of days. I'll try to post a picture when I get it on if you'd like.