the jackson hard counters the stug in the same way the su85 does: by having more range. IVs work fine but you have to be aggressive, which can be difficult to judge a good time for.
StuG's take 3 hits to get killed by an SU-85, a single Jackson hit reduces that to one hit from any vehicle or AT Gun. A two-hit wonder isn't really reliable, especially when the hitters tend to travel together. Panzer IV's on the other hand are more durable, but still suffer against Jacksons which kill them in 3 hits, making a once reliable unit as fragile as a StuG is normally.
From there, your only option is to go for Tiger Tanks (most common) or go for T4 Panther Tanks (less common, but my preferred). Yesterday I got a Tiger because I wasted my fuel on T3 and it's vehicles. It wasn't until after the Tiger was deployed that another Panzer IV I made could be useful, using the Tiger to deal and soak the damage and having the Panzer IV chase down the wounded Jacksons.