Making the Jackson a sniper tank is easy. You just have to change it such that it has to use its range advantage to win battles. See the Firefly and SU-85 for good examples of how to do that.
The hard part is doing that in a way that doesn't have far reaching and unpredictable consequen
It's a keystone unit for USF and any changes to it will have huge repercussions. It's understandable why the balance team is so hesitant to change it when each patch might be the last.
1) M36 is being OP for 3 years now (and I was pointing out even during the preview)...
2) If one want to stick with a "sniper" design why has to simply apply similar logic with "sniper":
Reduce mobility since a sniper is not faster than infatry
Use an inverted DPS profile similar to sniper DPS profile reducing ROF (or even accuracy and penetration) so that unit is an disadvantage when in close range
3) But there is little reason to use a "sniper" design since the part of the problem is that in the current implementation the unit has the mobility to stay at max range and little reason to move closer since its DPS is very high at max range and improves very little at closer range.
4) Having a units with the mobility of the M36, the accuracy of the M36 even on the move, the penetration of M36 at even at max range and DPS of the M36 is simply bad design. Something has to change.