I've got a 02/03 TJ Executive Wagon.

I recently installed a bit of dynamat in the back door to stop it shaking so much from my sound system.
Only problem is, it has added weight, and now the back door lifts won't open it.
When it is cold, they can't even keep it open.

They are called Stabilis LIFT-O-MAT gas springs.

Model: 9793GC 0510N 141/03

I was wondering if anyone has had the same problem?
Can you get upgraded ones to lift a heavier door? or would just replacing them work?

Anyone know where I could order some?

not sure where.. but I'm pretty sure you can get them in various lengths and force rates, but not sure how you would go with the correct ends on them to suit your car attachment points. Google stabilis lift-o-mat and gas struts / custom gas struts. Possibly about $20-30 each or a pair, we got some smallish ones in for work sometimes but I can't remember where from.

Not sure if will solve but I sell new ones, often the od ones don't take much weight

There is a mob on Ebay that have new ones that would be the same as Mals but if they actually make them you could contact them and ask if they can make ones that can handle the extra weight.

New ones would hold up a lot better.

I recently bought a pair for my TF wagon

$40 a pair. It was the cheapest price I'd seen. (Plus $15 postage to Adelaide). Total $55.

http://www.lifteasy.com/

"For a limited time only, mention this website and for only $40.00 a pair * plus GST, we will supply brand new gas struts for your vehicle."

(Click the CAR STRUTS > DISCOUNT CAR STRUTS tab.)

Ordering was a little bit of a pain. I had to email them (mentioning the website), explain what model I wanted, and then pay by Bank Transfer. It was tedious, but at that price I wasn't complaining.

A Magna back door is incredibly heavy. I doubt a bit of Dynamat was the only problem. The lifts were probably on their way out anyway. The dynamat just tipped them over the edge.
Its very easy to replace the lifts. You just need one or 2 spanners and a length of wood or a mate to hold the door open. You'll be amazed how much strength is still in the old lifts when you remove them. It really makes you realize how heavy the door is and how much pressure goes on those struts.