U joints? or driveshaft?

[Unit 5302]

You have a siezed or partially siezed U-joint. It needs to be replaced immediately before you bend or damage your T-5's tailshaft.

A loose d/s would cause a serious clunk along with a vibration, a broken tranny mount will clunk upon changes between acceleration and deceleration. Since you have no clunk, those aren't the problems.

While the driveshaft can cause a vibration from throwing a weight (technically possible), you will not notice it at low speeds.