Toilet or drain field problem??

I had my septic tank pumped a few months ago because water was not draining from my shower and the toilet would not flush after a shower. You had to wait awhile to flush. It was also a lazy flush all the time. Problem solved until now......Shower does NOT back up, but toilet gurgles when you are in the shower and makes a dripping noise coming from the tank. It is sluggish again and requires almost constant plunging. Rented a snake and snaked the vent....absolutely no change so I am stuck. Any advice would be appreciated......

(Have an oak tree near my drain field, but not sure if that is the problem....hope not due to the expense!!)